Not very often. Secret Service Baker is usually only active during special visits. FBI repeater is almost always encrypted. I sometimes hear clear activity on the DEA channels. US Marshall repeater used to be active for federal courtroom security, and it may still be, but I haven't monitored it in a few years.
A year or so ago I made a lot of effort to search the Fed bands and came up with a lot of unidentified fed frequencies. I think most of them were from Shaw AFB.