well the best way to do this is 2 filters and 2 centrifugal fans(a third one for your light/s), one scrubs and he other exhaust( their are ways to set it up with one filter, but it wont be any cheaper than buying another filter)
one filter scrubs 24/7 the other exhausts right b4 you fill up with co2 (this will serve many purposes for the time being understand it brings your co2 ppm to around 300) i usually estimate about 3 hours or less of useful co2 time, these factors will depend, you can but the simple tester kit and run schedule nd test it at different times of the day to understand how long your co2 last on average, and remember his will change with plant number.
i exchange air every hour and a half to 2 and half hours, im a little wasteful with my co2 but thats because i want to ensure my ppms are in range for the longest period of time, and my room isnt he best sealed.
http://www.hydroponics.net/learn/co2_calculator.asp
heres a link for the co2 calculator should prove helpful,
even if i where to get a controller i would still exhaust out the room several times over the day, the benefits from fresh air outweigh the cons from a sealed room, IMEO
test kit, youll have to but the refill tube they aren't cheap at this website but they have them cheaper other places,
http://www.hydroponics.net/i/131054