i mentioned this in another thread... i worked the to-go counter at a pizza/italian place for a few years and most of the time received $1-5 each transaction. on big orders, $100 or more, some people would tip around $10.
many i could tell just wanted to help me out, probably could tell i was a college student. having gone through this, i always leave $2-4 when picking up lunch somewhere.
$0 and anyone who does anything else is a sucker
you're already paying inflated sit-down restaurant prices for the food... absolute scam if they expect you to tip on top of that.