How long until my battery lifespan runs out on my iPod Touch?

For those of you who don't know what 'battery lifespan' is, it's the total amount of time your battery will last before it must be replaced. I use my iTouch every day and have to fully charge it every night. So from start to finish, how long would it be until my battery lifespan runs out? I've had my iTouch since February of this year.How long until my battery lifespan runs out on my iPod Touch?Its typically around 18 months. Just long enough for your warranty to run out. ;)