Once or Twice every two to three months. Otherwise you're nuts and playing roulette with the health of your lower bowel.
Apart from the obvious medical hazards of lesions to the wall of the intestine, you must remember that the tissues are not indefinitely elastic, they lose their elasticity under ' normal ' functions such as prolonged chronic constipation. Whereas a vigorous penile invasion once in a while exerts a shearing effect on the wall of the gut, it does not distend it unduly, and besides is more often than not very enjoyable. This shearing effect usually does no more than slough off squamous epithelial cells which are readily replaced.
Constant bowel obstruction can also lead to thickening of the gut wall, and if you've ever heard of Diverticulitis than read up about it. Most unpleasant and very bad for one's love-life ( even if you're Hetero )
Oh yes, and don't forget Anal Fissures and Gut Prolapse.
I could go on all night about the pleasures of Proctology, but my gut feeling ( Ha ! ) is that you'd get bored !!
So, fellers, if you're going to experiment with new sensations by fisting, do so with great caution, loads of Lube, and someone who is not going to treat you like a toilet that needs unblocking ! And moreover do these in-depth experiences a long way apart from each other. ( at least 6 - 8 weeks )
Now play if you must . . . . .
for me.