Describing himself as "weary" after two years as the Reagan administration's point man here, U.S. ambassador Deane R. Hinton has predicted a protracted Salvadoran war, partly because U.S. aid for the past two years has been "too little, too late."

As "one of those who think the war is going to go on for quite a while yet," Hinton added, "I have said for two years we need resources."

Similarly, Hinton said that a cleanup of the military's human rights abuses--a source of much U.S. congressional opposition to aid for the Salvadoran government--is "a question, in my view, of years," perhaps a decade or more until there can be what the U.S. envoy called "a generational change" in the officer corps.

On the eve of the speech by President Reagan to a joint session of Congress asking support for his Central America policy, Hinton pictured the conflict as also having been stretched out by Nicaraguan support for the rebels.

The revolutionary Sandinista government in nearby Nicaragua has "a great deal to say about how long the war goes on," Hinton told two reporters. But he said there is not yet enough evidence to determine whether counterrevolutionaries fighting Sandinistas in Nicaragua have had any impact on rebel arms supplies here. The stated purpose of U.S. support for the rebels is the "interdiction" of what the administration says are Nicaraguan arms shipments to the Salvadoran guerrillas.

A slight drop noted by intelligence sources in reports of shipments to the guerrillas could easily be "cyclical" rather than the result of what is happening in Nicaragua, Hinton said. "The evidence is not yet clear enough for me," the ambassador concluded.

Hinton, 60, has been ambassador here since June 1981. He has extraordinary influence, even in the context of a region traditionally dominated by U.S. interests.

Many diplomats and Salvadoran officials credit Hinton with personally stopping a takeover of the government by right-wing leader Roberto D'Aubuisson, a former Army officer, a year ago when D'Aubuisson put together a majority coalition in the then newly elected constituent assembly. A centrist civilian, Alvaro Magana, was named instead as interim president.

Hinton portrayed recent developments in the government here as generally favorable to U.S. policy. He praised Magana effusively and said it will be too bad when he is no longer president, after elections at the end of the year. The only "downside" of Magana coming to power, Hinton said, was that "one had to resort to the military" to put him in office in April 1982. With Hinton telling them a cutoff of vital U.S. aid would be likely if D'Aubuisson were to become president, the high command finally presented Magana as the only choice the assembly could name that they would accept.

"I'm moving them in the right direction," Hinton said. "Or, rather, they're moving in the right direction while I sit here and watch it." But Hinton, a former assistant secretary of state with more than 30 years in the Foreign Service, is about to take two months of home leave. Some of his prerogatives are to go to a special new U.S. envoy whose mission, as Hinton described it, will be "to wander around Central America and straighten everything out."

During his assignment here so far, Hinton said, his greatest disappointment has been the failure of the Salvadoran courts to convict the alleged killers of four American churchwomen and, in a separate case, two American agrarian reform advisers. The suspects in both cases are members of the U.S.-backed military.

One officer involved was released in October for insufficient evidence. That move prompted Hinton to make a sharp speech on Oct. 29 denouncing what he called the "mafia" here and raising the possibility of a U.S. aid cutoff. The ambassador was taken to task for this by some White House staff members, who leaked their criticisms to the press.

"They thought it was not in keeping with a quiet diplomatic approach. They thought it was a little loud, public diplomacy," Hinton said. "I thought the basic policy is quiet diplomacy but there is provision for exception. I decided the time had come to go public, and I went public."

Hinton said that the criticism from the White House was "basically" in the leaks and not in official cable traffic. "I think whoever went to The New York Times did the president of the United States a disservice," Hinton said.

Another area of considerable frustration, Hinton said, has been the crumbling Salvadoran economy. For three years, the embassy has hoped to see the economy quit shrinking but has not been able to halt the decline and does not expect to now, Hinton said. "Until recently, no one seemed to share my view of the magnitude of the effort that would be needed to stop the decline," Hinton said.

On the military front, the failure of Congress to meet administration requests for military aid earlier this year has cost 10 months in training the Salvadoran Army, Hinton said. Meanwhile, he noted, the war "is going on the way it has been going on. There's good weeks and bad weeks."

Hinton blamed much of the U.S. public's reluctance to support this government on what he called the "Vietnam syndrome" and a "deep-seated image" of Central America as "unstable banana republics run by repressive generals"--an image he said "doesn't fit."

In the wake of defense minister Jose Guillermo Garcia's resignation earlier this month and the crisis that surrounded it in the Army, Hinton said, "For the first time, you can say that civilian President Magana is the number one policy man in the country."

But Hinton insisted that virtually regardless of how deep the problems of the government that Washington supports here, the United States has virtually "no choice" but to be deeply involved.

"There is no question but what there is a conflict of interest when you are playing for big stakes and you don't want a Marxist takeover," Hinton said. But, he concluded, "We are not about to be beaten."

The armed forces here must sort out their problems internally, Hinton noted, adding that the essential "bias in the institution" is against the killing of civilians that has become common here in the past four years but has shown some statistical improvement in recent months.

"You're going to have to somehow keep democracy and the economy going here for 10 years, for a generational change," said the ambassador.

Asked finally if he wants to return to El Salvador when his leave is over, Hinton said, "I'd like to see it through in one respect, and in another respect I think, you know, you get burned out."

When asked, however, if he was "burned out," Hinton replied, "No, basically. No."