Google limits the maximum duration of a script to 6 minutes. That means you can only find between 300 and 500 email addresses at one time, then re-launch it manually.
For the same reason, it isn't possible to select a list of more than 500 domains for the bulk Domain Search on the Google Sheets Add-on. Even below this number, the script could potentially time-out before all the domains have been searched (especially when requesting more than 10 emails per domain).
If you have any issues or questions about Tomba, feel free to contact us, we will be happy to help you!