I know this usually goes without saying. I just want to make sure everyone knows that when you administer a drug rectally, it has to be in the form of a solution before it's administered. To just take a pill and stick it up your rectum would likely lead to a partial absorption of the pill/tablet i don't know, maybe 10%-20% before the majority would be cleared out the next time you defecate.
Anyway, if you have your pill/solution, which should ideally be less than 10ml of solution, you will absorb the majority of it within 15 minutes following administration. It's a pretty quick process really. Back in the day I'd be feeling the full-force of a dose within 10-15 minutes.
I'll say that in years of usage of this ROA as a secondary option, I've never actually had anything come back out. Some people apparently have issues with this, but I've never had this issue.