Co2 useage and tank life

So I have a 1150 cuft grow room. My co2 tank is 20lb. The math says to set my solenoid to .26 How long will it last ? Also do my small plants need 1500ppm of co2 or should I wait untill flower ? I am vegging under 1k mh and will be flowering under 2k hps. The ballasts are lumatek and lights are digalux. The co2 set up is Active air co2. I am growing with bag TGA base and supper soil. I feed with just water and carbs. Temps are 75 to 78 all day.

Looking for any input, this is my first room build and first indoor grow. It legal and I ran out of money so no air filter or mylar yet. All of my clones are 3rd floor blue dot in 7 gallon pots.

Thank you
Darth Dank

Get used to going to the gas store

it's good to have 2 tanks. how long it will last just depends on your room. i never use co2 in veg...

In my 12x16 ft flower room, completely sealed, no intake or exhaust with 4-1000 w hps lights and 40 plants a 20# tank would last 2 weeks when set at 1500 ppm. I use a preset 1500 ppm meter to control the solenoid on the tank. I don't usually use co2 in the veg room. Occasionally I put some beer that I brewed in my veg room and ferment it in there to produce co2. HAhah.

Tank life will depend on how well your room is sealed and how much foliage is in the room. I used to get 4 weeks out of my first tank then 2-3 per tank with a room full. If your room isn't sealed and you are exchanging air with the outside then I'm of the opinion that you should skip co2 and exchange air frequently.