you don't want to just have it leaking. You'll probably get toxic co2 levels. You need a regulator on a timer. Or if you have exhaust fans running 24/7 then the co2 most likely won't help at all.
Yes, I will get a regulator & timer so that it only leaks during Photo periods. I just want to know how long a 20 lb tank would last. I'm hoping at least a month while providing at least 1500ppm, if not I will get a bigger tank. If my grow tent is 3.5' X 2' X 5' and I get a fan to refresh air inside once every minute, how much C02 do I need to release p/sec or p/min to get 1500-2000ppm?
Alot depends on your temps and vents. I go through two tanks every 70 days during the hot months. I only use a single tank every 70 days the rest of the Year.