First of all, this should be in Suggestions/Request.
You can run tasks in your plugin_init().
Something like:
Code:
set_task(get_timeleft() - 300.0, "Hook5MinutesRemaining");
For 5 minutes or
Code:
set_task(get_timeleft() - 30.0, "Hook30SecondsRemaining");
For 30 seconds.