Staff departures are accelerating, major policy announcements are decreasing and the hot topic of hallway gossip is about future job plans, not current political intrigue. The White House is becoming a lonely place.

By the calendar, Ronald Reagan has seven months left in his presidency. But by political reality, the curtain is coming down on his administration.The symbolic transition to a caretaker presidency occurred this week when White House chief of staff Howard H. Baker Jr. announced he would resign at the end of the month to tend his ailing wife, and Baker's protege, White House communications director Thomas C. Griscom, said he was resigning to seek private employment.

Both men timed their departures to come on the heels of Reagan's Moscow summit and next week's economic summit in Toronto, the last major events of Reagan's presidency.

With no other important presidential trips or activities on the schedule and no new initiatives for Reagan to push, Baker and Griscom told colleagues they felt comfortable leaving.

Reagan named Baker's deputy, Kenneth M. Duberstein, as his new chief of staff, another sign that his presidency is winding down or going "on automatic pilot," as one adviser put it.

The 44-year-old Duberstein is highly regarded as a detail man who keeps the White House functioning smoothly on a day-to-day basis but lacks the stature of his three predecessors - Baker, the former Senate majority leader; Donald T. Regan, the former Treasury secretary and Merrill Lynch and Co. chairman; and James A. Baker III, the current Treasury secretary.

"We're down to the janitorial work," observed one White House official, who said he and his colleagues are thinking increasingly about their own post-administration futures. "Basically, people here are making sure the lights are turned off, the doors are closed and the floors are swept."

One official said first lady Nancy Reagan is "totally preoccupied" with her move back to a private life in California next Jan. 20.

The Reagans will lease a home in the fashionable Bel Air section of Los Angeles, where Mrs. Reagan will be busy with plans for a drug rehabilitation center and Reagan will be involved with his presidential library. "All she talks about now is the move," said a friend, who added that the president also talks often about his retirement plans.

Mitchell Daniels, Reagan's former White House political director, said the remainder of Reagan's term will be dominated by the Democratic and Republican political conventions, the fall presidential campaign and the transition to a new presidency.

"It's too late to start any new legislative programs, and in the diplomatic areas, there's not much there with the close of the economic summit," Daniels said.

Aside from an unforeseen foreign crisis that could suddenly revive White House activity and put Reagan in the spotlight again, the agenda of the White House team that remains is to see to a conclusion the foreign and domestic policies whose outcomes already have been determined by Reagan or Congress.

"Frankly, the White House is a big bore right now," said one senior official at the Commerce Department.