SANTA BARBARA, CALIF. -- President Reagan will pass a milestone during his 25-day vacation in California, most of which he is spending at Rancho del Cielo, his 688-acre "heavenly ranch" high in the mountains northwest of here.

By the time Reagan returns to Washington Sept. 6, he will have spent more than a year of his presidency in California, where he served as governor for eight years and plans to live after leaving office in 1989.

When Reagan completes his current trip, he will have been in California during 374 days of his presidency, including 290 days at the ranch. Most of the other 84 days have been spent in the Los Angeles area, which Reagan has called home for the half-century since he went to Hollywood as a young film actor, or in Palm Springs, where the Reagans vacation each year the week after Christmas.

Reagan will be in Los Angeles for three days next week, where he will give a speech on U.S.-Soviet relations and hold meetings with leaders of the Nicaraguan contras and supporters of his Supreme Court nominee, Appeals Court Judge Robert H. Bork.

Frank C. Carlucci, the president's national security adviser, encountered the usual journalistic skepticism last week when he tried to explain administration policies in the Persian Gulf and Central America to reporters during Reagan's trip to California.

But Carlucci has made believers of the White House press corps on the tennis courts at the Santa Barbara Biltmore, where he has been routing reporters and members of the White House staff with a game that one victim described as "awesome."

Carlucci, small in frame but highly competitive, plays tennis and swims a mile each day after completing his daily duties in a cottage on the hotel grounds where traveling members of the National Security Council staff are stationed while the president is at the ranch.

This year Carlucci is being housed in the Ortega Cottage, which hotel personnel have hastened to explain has no connection to Nicaraguan President Daniel Ortega. The hotel's director of marketing said that the name Ortega, a common one in Santa Barbara, was chosen to "reflect the area's Spanish heritage."

It is an axiom among Reagan's aides that the president prefers telling anecdotes to working, and his behavior apparently hasn't changed much over the years.

In the current issue of Santa Barbara magazine, Ralph Nelson, who directed Reagan for television's "GE Theatre" series, recounts how "Ronnie" liked to regale the cast with stories during breaks in rehearsals. When Nelson would try to get the cast back to work, Reagan would say, "Just a minute, Ralph, let me finish this story . . . . "

Nelson chatted with the president last year at the annual barbecue hosted by the Reagans for the traveling White House press corps and the press office staff.

True to form, Reagan told Nelson a story about the time his pants had ripped moments before broadcast time and a "little old lady" from wardrobe was called in for a quick repair job.

"Suddenly we were on the air," Nelson quoted Reagan as saying. "You had a closeup of me introducing 'GE Theatre' while this little old lady was on her hands and knees stitching up my trousers."

Santa Barbara, best known for solitude and sandy beaches, has taken relatively little notice of the Reagan presidency during the past seven years. But the annual media barbecue hosted by the Reagans at a nearby estate at Hope Ranch has become so popular and unwieldy that the White House staff this year tried new strategies to reduce attendance.

"We were reaching the point where we were being inundated by friends of friends and friends of their friends who drove up from Los Angeles for the weekend," one aide said. "Last year we served 700 meals."

To keep down the crowd at the annual beef-and-beans affair, White House officials held the event yesterday afternoon, instead of the usual Saturday late in the holiday trip. The tactic worked perfectly. The turnout was about half of what it had been in 1986 despite perfect Santa Barbara weather, with temperatures in the low 70s.

The ground rules for the event -- the only time reporters can talk directly to Reagan -- remained unchanged, however. They provide that anything said by the president or Nancy Reagan is off the record and they prohibit photographs of the president.

White House spokesman Marlin Fitzwater's daily briefings here have been distinguished by an absence of news and an exchange of one-liners between Fitzwater and the press corps.

On Wednesday, a radio reporter became disgusted with Fitzwater's daily report that Reagan was clearing brush at his ranch each day and asked, "Is a new load being hauled up there every day for him to clear?"

In yesterday's briefing, Fitzwater was peppered with questions about a report that senior administration officials had in 1984 approved a plan under which South Africa would provide aid to the Nicaraguan contras. As is usually the case with serious questions, Fitzwater had no comment and said he didn't know if Reagan was aware of the plan.

But he said in response to another question that the administration currently favored third-country attempts to help democratic groups in Nicaragua by any organization, including the "Garden Club of Santa Barbara."