As of Patch 1.9 or so, players are limited to entering 5 instances per hour. Meaning that if you start your first run at 1:00pm, and do 4 more runs within an hour, you will not be able to enter any more instances (aside from Battlegrounds) until it is 2:00pm. An addon like Loot & Reset can illustrate this to you and help you time your runs.
- 1:00 : Instance 1 entered
- 1:10 : Instance 2 entered
- 1:20 : Instance 3 entered
- 1:30 : Instance 4 entered
- 1:40 : Instance 5 entered
- 1:50 : Instance 6 Unable to enter. You will simply walk through the portal without entering the instance.
- 2:00 : Instance 6 entered
Note: The number of instances you can enter is linked to your other characters on the same account as well. This means that if you have reached the entering limit on character A, character B will not be able to enter any instance.
Time Limitations (Heroic)
You can only enter each heroic instance once per day. Heroic instances reset at a specific time each day (9am server time on Proudmoore). Players inside an instance during reset time receive a prompt telling them that they will be saved to the instance again, with the option to teleport out. Clicking "Accept" or remaining in the dungeon for more than sixty seconds from the time the prompt is displayed will save players to the instance again, as would extending the instance lock. It is possible to enter same instance more than one time if you get there via random instance.
Some instances are limited to a party of 5. These are considered dungeon, party, or just plain instances. Some others have a player limit of 10, 20, 25 or 40 players. These are considered Raid instances. Most raid instances will not allow you to enter unless you are in a raid, the exception being Blackrock Spire, which has two areas, one of which is intended to be done as a 5-player party, the other as a 10-player raid.
Instances are designed for a certain character level - mainly because of the level of the mobs inside. Higher level mobs will crit your tank more often(tanks become crit immune after 13 talents points spent), dodge more attacks and resist your casters' spells also any mob that is higher than 4 levels of you can do crushing blow. Lower level mobs, on the other hand, may not pose enough of a challenge and will give you little to no experience.
The Looking For Group tool shows the instances suggested for that level.
Each instance also has a minimum required level to actually be able to zone in, however this is considerably lower than the suggested level. Both these level requirements can be checked at the Instances by level article.