THE AMERICAN ECONOMY now seems likely to give President Reagan a grace period in early 1985 to establish policy for his second term. It will be the administration's opportunity to deal with all of the fundamental questions that, for the past 18 months, it has evaded in deference to the election. A variety of forecasts -- of production and demand, of the financial markets, of the currency exchange rates -- all indicate a fairly low risk of disruptive changes until some time toward the end of 1985. But after that, if present policy remains unchanged, the risk can be expected to rise sharply.

The reason is that the American economy is now badly out of balance. For the present, things are going along pleasantly enough. But there are clear signs of trouble ahead. There's the administration's gigantic budget deficit, with the government now spending five dollars for every four that it raises in taxes. Beyond that, there's this country's rapidly rising deficit in its accounts with the rest of the world, as Americans buy three dollars' worth of imports for every two dollars' worth of exports that they can sell. That's related to the extraordinary overvaluation of the U.S. dollar abroad, trading at prices one-fourth higher than its worth in terms of the goods that it can buy. The whole structure of present American prosperity is based on an unprecedented inflow of foreign investment, currently more than $100 billion a year. That won't last forever.

But it will last long enough to give the president time to seize the initiative -- if he wants to. Unfortunately, the air of serenity that prevails in the economy currently has drained the sense of urgency out of the discussions about where it's headed. Three years ago, when the budget deficit was moving toward $100 billion a year, there was something close to panic in and around the administration. Now that the figure is hovering just under $200 billion, people yawn and say that it presents many interesting theoretical questions.

It's a reasonable guess that Mr. Reagan has six or eight months in which to assert his control over the money questions. That has to begin with a credible -- repeat: credible -- commitment to reduce the budget deficit, bringing down interest rates. With a gradual decline in the dollar's exchange rate, the country's international accounts will improve and eliminate the present dependency on foreign investment. Done skillfully, it would permit continued rises in incomes and standards of living.

But if the president lets this period of opportunity pass, the financial markets will eventually begin to resolve the imbalances in their own way -- overreacting wildly and destructively, driven by speculative surges. The president has time to act, but not a great deal of time.