Improve how to limit a loading session

Limiting only by a maximum number of emails per server, is a bit limited.
When used with a scheduler, it would be more easy to limit to a certain duration.
For example if I schedule a load every 15min, I would like to limit in the session to 15min (or a little less, as it would be difficult to limit to exactly 15min).
Implement it through additional field ("maxDuration"), with a static list(key=time in minutes, display=some human durations like "infinite", "1min", "5min", "15min", "30min", "1h", "1d")