it really depends on your usage. if you force carbonate, it will take a lot more co2, if you use priming sugar, and only use co2 to dispense, it will last a lot longer. if you get even a small tiny leak, it will last you a day maybe. haha. check everything for leaks when you have it assembled. and by check, i mean pressurize everything, and then turn off the co2 at the tank. go to sleep, check the next day if the high pressure gauge has moved any. if it has, you have a leak, you can also check for major leaks with the soapy water way, but still do this to check for small leaks, because even if it is a small leak, you will go through co2 very quickly. the reason you check the high pressure is because there is a lot of pressure in the regulator, even if you close the tank valve, and this high pressure will continue to keep the regulated low pressure even if there is a leak, until it runs out. if it shows empy and no pressure in the low gauge... you have a pretty large leak.
i hope that made sense.
also, it is pretty cheap to swap co2 tanks, that is unless of course you are really attached to your nice shiny spun aluminum tank... then it may take a few days/week if you get it done at a welding supply shop, or usually more money if you get it filled on the spot.
have fun with the kegs. i was not sorry when i left bottling behind... heh