I usually heat to boil in the microwave, and stain for 12 minutes. How often can I reuse this buffer?
Since there's a lot of methanol in Coomassie stain, a significant amount would probably evaporate off each time you microwave it. Therefore, it'd be probably a good idea not to reuse it because (1) you're losing methanol, and (2) the concentration of everything else is thrown off.
Why do you boil the stain? I just pour mine at RT right on the gel and it works fine.