For every person that says “the posted wait was 20 and we waited 10”, you’ll find someone else that says “we waited every bit of 20 mins.” Party crowds can be lumpy and ride waits (posted and actual) can ebb and flow over the course of the night, often influenced by the two parades and fireworks spectacular.
Generally speaking and depending on the ride, my experience is that they are usually inflated 5-15 mins. For example, BTMR says 20 mins right now - my guess is it’s probably 10-15. Splash day 10, it’s probably close to a walk on. But that’s not some hard and fast rule... experiences can vary.
The good thing is that with no FPs, to me lines “feel” like they move quicker since you’re generally constantly moving.