User blog comment:Numinous J/Custom Champion, Ruaidhrí/@comment-26815373-20151122185607/@comment-26378838-20151122214107

The lifesteal cap is included in the "ability details" tab: it's 200% (which also caps Cannibalism at 80% and his ult bonus damage amp at 240%).

It might be seem high, but considering that in a normal game and with a non-cheese build he'll most probably only reach 50~60% lifesteal (and, by proxy, 30% on Cannibalism and 70% on his ult amp), he needs over an hundred kills+assists to get to the cap, unlike the likes of Nasus, who can reach great stacks by just farming minions.

Speaking of Nasus, I'll point out that Akali has a spell vamp passive just like Morgana, but they work differently because Akali needs to itemize properly to make the most out of her passive while Morgana's a clean number and she doesn't need to work for it. Similar here: Ruaidhrí needs to score kills and/or assists to build up his lifesteal, while Nasus has a clean number that he doesn't have to work around for.