24/7 - You need that refresh of air, especially co2, which the plants need more of.
It keeps things cooler, and reduces chances of mold, when you are in flower.
Fresh air in and stale out always is the best. If you want it for heat control then you can hook it to a temperature switch and set the degrees and away you go.
If the smell and humidity / heat are not an issue you should have no problems doing this. I'd say run a test before setting and forgetting. Make sure everything stays in acceptable range! Cheers and happy growing!