atLEAST 1500fpm, but 2k or greater is ideal. The guys that give a 500fpm descents off the bat, and then when atc asks to increase rate of descent they go up to 7-800fpm ruin it for everyone else often, and make the controller issue blanket descent rates since the guy earlier in the day burned him or really increased his workload.
In my opinion, only time you should be descending at less than 1500fpm (2000fpm ideally) is one of three things: descending with PD, meeting a restriction, or about to level.