So a 10lb co2 tank is big and unless you have an incredibly large tank should last months maybe 3-9 months or more depending on flow. Most people have a bubble rate of about 30-75/ minute. I have a 180 gallon tank and I am running about 60 bubbles per minute. Both the bubble rate and effluent rate determine the amount of dissolution of the media. Some people use an internal pH probe to determine dissolution rate as well.
So things to check leak in:
1) regulator to co2 tank. You can turn off bubble flow, close off the regulator, turn on the co2 tank and note the pressure (usually around 50-55 on new co2 tank), then turn knob to off position on co2 tank and after 15 minutes or longer, you can check pressure on regulator . If there is no leak, pressure should not have dropped, then you have a solid connection between regulator and co2 tank. This is probably the most common leak (the most likely culprit) because of the crappy gasket between regulator amd co2 tank
2.) make sure you are using "co2 tubing" between the regulator and your reactor, anything else will fail on you. Check to make sure you have a tight and sealed fit/connection with the co2 tubing on the regulator and on the reactor. The only way I know how to check a leak there is with some dish washing solution applied to the outside of the connection and see if there is bubbling. Be really neat because you don't want dish washing soap in your water anywhere.
And if you suspect a leak, it is almost always #1, a leak between the regulator and co2 tank
I get my co2 refilled at a medical gas supply house, Leeper brothers. I am sure they fill them very precisely at Leeper Brothers.