President Regan settled into his familiar vacation routine at his rance today as he bagan one of the longest absences from the White House in modern presidential history.

Regan does not plan to return to Washington until Sept. 32, a 28-day stretch that amateur record-keepers say is topped only by President Nixon's 30-day absence from the capital in 1969.

The president rode horseback with Nancy Reagan today and cleared brush at his 688-acre rance in the Santa Ynez Mountains about 20 miles north of here. So familiar is the tranch routine of clearing brush and chopping wood, that when White House deputy press secretary Larry Speakes announced that the president would clear brush today, Speakes joked, "I guess tomorrow it will be chopping wood."

Most, but not all of Reagan's California stay will be vacation. The president will spend a week in Los Angeles, where he is expected to hold a news conference. He also plans to attend three Republican fund-raising events organized to raise $500,000 for California local election campaigns next year.

Next week, the president will interrupt the near total privacy he and his wife enjoy at the ranch for a ceremonial signing of the budget and tax bills, which were the first priorities of his administration.

Reagan is being kept abreast of national security developments and other matters, including the air controller's strike, by telephone and with documents carried by his personal aide, David Fischer, who suttles between the ranch and a temporary staff office in Santa Barbara. Only White Hose doctor Daniel Ruge, a military aide and Secret Service agents are permanently at the ranch with the Reagans.

From nearby mountaintops the television networks maintain a watch over the president with long-lens cameras that produce pictures showing tiny figures moving about on the ranch below. CBS has added a starlight scope to its camera so that its surveillance can continue after dark.

At the daily White Hose press briefing transplanted from the ywhite Hose to a meeting room in a beachside Santa Barbara hotel and attended by reporters in blue jeans, shorts and bathing suits, Speakes had little to announce.

Press secretary James S. Brady continues to recover well in George Washington University Hospital, Speakes said. The prime minister of Thailand is to vist Washington and meet Reagan on Oct. 8, Speakes announced.

He also announced that Secret Service agents have challenged the press to a softball game and that bridge players are seeking more of their kind.

It was the last presidential briefing for Jeremiah O'Leary as a Washington Star White Hose reporter, and Speakes presented him with a tray of coffee and sweet rolls, which have seen him through many days in the White House press room. A chorus of "For He's A Jolly Good Fellow" ended the briefing.

O'Leary noted that nine minutes earlier the Star's last deadline had passed. He surveyed his colleagues and remarked that he was the only reporter in the room who had to make no pretense of working