I'd say no more often than once a month, and no more than .5 grams that night that you do it. Also, don't even think about smoking it or anything other way of using it other than insufflating.
I am still sickened that I technically smoked crack on friday night after a evening of blow (made a bit of freebase coke myself by just adding baking soda and water, was only about 30-50mg but man looking back at it soberly now, wtf is wrong with me.) Drunk and coked out me thought it would be such a good idea to try to smoke some crack...
wth man!
Anyways, so yea definitely no more than once a month, if not once every 2 to 3 months.