The most common cause of a maximmum execution time timeout error is an overly large category selector. If you get the error after (or while) submitting or editing a listing, this is likely the problem. To solve that, first go to Admin Panel -> Settings -> Switches and make sure you have the level-based category selector turned on. Then go to Admin Panel -> Miscallenous -> Advanced Options and run this query in the Run SQL box:
UPDATE {PREFIX}settings SET content=0 where name='maplevels' or name='selectorlevels' or name='submitselectorlevels';
You should find the problem solved after saving that change.

It can also be caused by attempting too many static exports or javascript exports at once, or by using automated backups with a database that takes too long to backup. Any page which attempts to display too much data would encounter this same error.

It's possible, though less likely, that you've placed so many toplists in a template that it can't load within 30 seconds.

t's also possilbe, though unlikely, that you could be trying to show hundreds or thousands of links on a single page and that this would cause it.