Absolutely 24/7; my 6" max fan uses 48 watts, for the whole 24 hours that's only 1152 or 1.15 kWh. (On low speed). PS I'm exhausting two 4x4 tents with that.
If you run it for 16 weeks, 8 veg, 8 bloom it will only add $21.93 for electricity, (I am basing that on $.17/kWh, if your not in California you probably pay less).
Question, if you don't run it how much does it cost to fight PM, and how much of your harvest is lost in the process? How much is a good crop worth?
My last run total electricity costs were $356. Last time at the dispensary I could buy a no history top shelf oz for that same $350.
Another thing, you could move that fan and charcoal filter out of the room to give you a little more space. What's the second inline fan for?