Agreed, with a 2-week lead-time having 2 cylinders might be helpful. Either that or start weighing your tank and see if the air gas can do an order-ahead for swapout when you are getting low.
Determining how much CO2 is in your tank:
If you want to really know how much CO2 is in your tank, use a scale and weigh it.
The cylinder should have its tare weight stamped on it, which is the weight of the tank when empty. Subtract this from the current weight and you have the weight of CO2 in the cylinder.
The "pounds" rating of a CO2 tank is how many pounds of CO2 they hold. A 5 lb cylinder might have 7.5lb tare weight, and would weigh 7.5lbs when empty, and 12.5lbs when completely full.
If your cylinder lacks a tare weight stamp (very unusual) you can weigh them when you get them, and work backwards ie: if you get the cylinder at 12.8lbs, it is going to be empty when it hits 7.8, roughly speaking. There's always a chance of slight over/under charge, so it might not have exactly 5lbs of CO2 in it, but it should be close enough for estimation.