I have a cat with IBS on my lap right now, and we had another one who passed away about 10 years ago.
For both, the prescription food was enough to control it. Going back to regular food would generate issues, and if the earlier one got into something he shouldn't, he'd often toss his cookies.
Ours weren't diarrhea issues, though. Both of ours were throwing up a lot. The earlier one, even with the prescription food, threw up a lot - even now with three cats, I still have less cat vomit to clean up.
Eventually, the cat that had IBS developed cancer and passed away. I had had him 12 years at that point, and he was a young adult when I adopted him (they said he was "about a year old" but a vet later told me that means "anywhere between 1 and 6 years old"). So he might have been as young as 13 or as old as 17.
(Same deal with lap cat - they told us he was about one, we adopted him in 2016, but his black fur is starting to fade in some spots on his legs, so I suspect he's older than they claimed. No real way to tell though.)