accurate estimate on the length of time before refill is impossible to tell, because one bubble a second and two bubble a second makes big difference, time before refill shorten by half.
and a four bubble per second on 20lb is about the time before refill as one bubble per second on a 5lb co2 tank.
there are things that are for sure:
1. put the 20lb outside for safty.
2. make sure there is no leak, some slow leaks that can not be detected by soapy water will shorten the timel; bigger leaks that can be detected by soapy water if left ignore, will dramatically shorten the time before refill.
3. weight your co2 tank so you will know how much co2 left.
4. you can complete list 4, 5,..., if I skip anything,