You can usually get an idea of how long they'll last from the box it comes in or the manufactures website. The box should say some thing like 2000 hours usage etc (maybe more, just a random number). Then just take out a calculator, enter how many hours a day you leave them on and divide it by the number of hours the box says and it'll give you how many days worth of lighting you can squeeze out of them. To be on the safe side, I usually go by half that amount. As we all know, some manufactures embellish a bit on their boxes.
"If your wife ever asks how much something was for your tank, just tell her you got it on sale and you had to have it! She'll understand"