heroics usually reset at 6 or 7 AM server time, if i recall correctly. be aware that the random dungeon tool ignores those locks. sometimes if you choose to do the random dungeon you'll get thrown back into one you've already done that day. --- You guys keep comparing this to real world problems. I didn't kill anyone, I'm not addicted to drugs, and I didn't stuff a pinata with broccoli. - Tanque
Heroics reset every day at around 6 AM server time now (used to be 8 Server I believe). So once you do a heroic, you won't be able to do that heroic again until that time the next day, or unless you get it as a random heroic.
Raids reset every Tuesday at 6 AM server as well I believe. So once you do a raid, you won't be able to do a fresh run of that raid until the following Tuesday after maintenance. --- Each person might see a rose with a different shade of red, but the fact that we can all agree on is beauty -- that is a miracle. -Michea, FFXI
When you pick a specific Heroic, it locks until roughly 8am the next morning.
10/25 Man Raids lock for a week, which I think turns over on Tuesday mornings. IE, if you ran tonight it would be open again tomorrow, but if you ran it again tomorrow, it wouldn't be open again until the following Tuesday.
The exception is if you get lucky with Dungeon Finder, it ignores instance locks (but, apparently, tries to avoid picking instances that have been done already) --- 'This would be my cue to say something snarky, wouldn't it? I'll have to remember that. For the next time you make an idiot of yourself...'