on a 240 litre tank a 500g bottle lasts me 2 weeks!!!!
that is under 1t8 and 2t5's.
if you are running high light i'd stay away from dissposable bottles and look into getting a fire extinguser set up as you can get these filled for as little as Â£10.
it costs me 12X as much money to run dispossable over fire extinguishers
using an FE may seem daunting at first but it really is no different, you still have to swap the regulator over no matter what bottle you have.
there is a formula somewhere on this forum to calculate co2 useage, but different bubble counters will produce different size bubbles and until you know the actual bpm you will require then it will be hard to get an exact figure.
co2 usage will depend on plant mass/types, also surface agitation will play a part and so will the difusion method i guess.