I don't think this is a Limesurvey issue. Even if you set a limit in Limesurvey, this limit might be ignored/get overwritten by global webserver settings.
The timeout usually works correctly, this is being confirmed by users complaining about running into timeouts.
If you still think it's a bug:
Please test if the problem still exists at
the latest release. If so please
open a ticket at the bugtracker and...
1. Give as much information as possible
2. Attach your survey. You can mark your ticket "private" so that only the developers will be able to take a look at it.
3. Provide step-by-step instructions how to reproduce the problem.
4. If possible/helpful please provide screenshots
Afterwards please post the link to the bugtracker so we can follow the progress.
Thanks!