A federal judge found yesterday that it takes the D.C. Department of Human Resources an average of 100 days to replace lost, stolen or mutilated welfare checks. He ordered the agency to develop ways to replace the checks within a "reasonable" time.

Judge Joseph C. Waddy made the ruling at the end of a hearing in which evidence was presented that the U.S. Treasury had proposed a replacement plan to DHR last September, but that then DHR Director Joseph P. Yeldell had not responded to it.

The ruling came in a suit filed by the Neighborhood Legal Services Program on behalf of welfare recipients. Gerald Von Korff, a lawyer for Legal Services Program on behalf of welfare recipients. Gerald Von month of the 32,000 that DHR sends to its clients.

Judge Waddy gave the District government until June 1 to present to him a plan for replacing these checks within a "reasonable" time. There was no indication of what "reasonable" might mean in this case.

Von Korff said that Boston, whose size is comparable to the District, replaces lost welfare checks within three or four days.

According to Von Korff, yesterday's testimony showed that the Treasury, which writes most of the checks issued by the District government, proposed last September that lost checks be replaced immediately. In cases where the checks had been stolen and then cashed, the immediate replacement would be followed by a Secret Service investigation to determine that a theft actually had taken place.

This is the proposal to which Yeldell failed to respond, according to the testimony.