From what I recall the bioavailibilty of whey is pretty high.
Absorption rates for isolate are quick, but you can slow that down by adding fats/fiber...etc
I typically buy a whey protein isolate/concentrate mix and typically use it for pre/post workout. I try to use a casein powder for other times but if I don't have any I'll just make sure to add extra fats like coconut milk/nut butters or milled flax seed.
"And for crying out loud. Don't go into the pain cave. I can't stress this enough. Your Totem Animal won't be in there to help you. You'll be on your own. The Pain Cave is for cowards.
Pain is your companion, don't go hide from it."