You'll get 100 answers on this all different. My understanding is that the biggest variable is the lube, depending on the type it significantly changes how long a clean bore will take to "settle". Ammo with little to no lube might settle in 5-10 shots, and I've heard guys saying with some wetter softer lubes it takes 50+. I'd imagine the best way to know would be a chrono, clean the barrel and shoot till velocity settles for the ammo you're using.
If I'm range testing lots of ammo types, I normally run a bore snake with some #9 through between ammo brands, and shoot ten, 10 shot groups, reviewing the groups usually tells you when the clean barrel is settling down. Normally for me I see this happen in the first 1-4 groups. Some of the really messy lubes, the stuff that leaves your hands, mags, chamber, and everything else wet/greasy almost always take the most rounds to settle in. I've also noticed that if you don't clean and just switch ammo, it takes longer to settle down.