They'll last a long time if you don't crush the beads, since Adderall XR has a time release mechanism...some of the beads release the amphetamines immediatly, then the rest will be released about four hours later.
Some may prefer it this way sure, but the beads arn't really hard to crush if you put paper on top of them (or even place them in a baggie) and use the rounded design of a lighter to your advantage. A razor blade can then be used to refine it, I have managed to do this within five minutes in a classroom.
If you want a stronger rush while helping to retain your ability to get to sleep within the next 24 hours, crush away is all im saying its up to you.