It varies, eventHandler; spells like Blass last a minute (ten rounds in combat), for example. The issue right now is that the spell information we're given is woefully lacking. It doesn't tell us nearly enough on almost any front.
A lot of the changes to 'last until you rest' are actually okay, or good 'for-video-game' changes. For example, mage armour lasting until you long rest (unless otherwise dispelled), is a buff to the spell's normal duration, and is a valid and worthwhile tweak for a video game. Sleep's duration being decreased to two rounds, on the other hand, is not.
If you look throug the spell focus thread, I point out most of the places where durations have been changed from the 5e rules, in case you just want to check for your own benefit.
