The length of the IP contracts vary a lot, seemingly depending on how big an investment the attraction is. They'll have certain clauses and the ability to have pre-determined extensions triggered.
I believe Saw was on an initial 5 year contract and had regularly been extended. The Lionsgate and Walking Dead mazes, and I'm a Celebrity, were 3 year contracts, with options of 1 year extensions. Angry Birds I've no clue actually. Walking Dead The Ride is under a separate contract to the mazes. I'd have thought 4 years, but who knows. But yeah, it's very true that it would be easy to retheme it to a generic zombie ride if they wanted to!
Specifically for Derren, I've heard the contract was 6 seasons in length, which would put 2021 as the last season potentially. But all that remains to be seen.