With Dexedrine, it's pretty much "Do you have school today? Should you wake up before you want to wake up?" Then, I do take a Spansule. Though, I also take dexamp when I'm too sedated by alcohol and cannabis and need to stay up and walk my sleepy ass back home.
With cannabis, I vape or smoke pretty much everyday that I possess it, though I try (not very hard) to wait until it's dark. The conditions I need there are: no immediate responsibilities, can't have smoked in the last 6 hours (less rewarding), and don't be in company you'll feel awkward around when high. I've been trying to only buy an eighth at a time recently, as it's easier to wait 2-4 days before buying more than it is to not vape stuff I already have in my room.
Opioids: I usually have taken no less than 3-4 weeks since the last high. Beyond that, availability, price, and the need for it to be a pharm (other than Darvocet or Tram) are the only factors. I'm afraid of tolerance and they seem like the most comforting drug, so my anxious ass doesn't need to risk a terrible addiction by finding a steady connect.
EDIT:
With alcohol, it's been extremely often since I recently turned 21. I hope to make it a few times a week. But, so far it's been every day that I want to relieve stress after classes and work, every day I want to prevent MJ paranoia, every day I want to be friendly with someone I've just met. I'm quite anxious, so a legal, cheap GABAergic is obviously going to be very tempting when the liquor store's 2 blocks away.