Alright, let me sum it up =)
Adderall XR (extended release) will last 7-9 hours, with after effects lasting a good 3-4 hours after until baseline. Adderall XR releases half the dose immediately and half the dose 4 hours later. IMO I can't really tell when the other half is released. So really, 20mg XR = 10mg IR
Now IR, its much more euphoric but has a MUCH harder crash/comedown. It lasts about 5 hours, from person experience. Adderall XR is a much cleaner, longer lasting high, but you must take more. DO NOT rail, it has a HORRIBLE sickly sweet drip, you will puke haha. I personally cannot handle the taste, and I've railed coke a few times.
-flap