My latest one about a year (since getting the iPhone), prior to that i had my old one for at leats 10 years. Only reason i moved to a new number was that the contract on my old number was still going, and i did't want to shell out to finish that off tp pac to my new phone.
On the subject though, i have to say that my kids are the most annoying when it comes to cell phone numbers, my daughter seems to have around 4 sims which she constantly swaps when credit suits.
Why the hell can they not get one card and blody well stick to it !? Means i gotta text all 4 numbers just to get a message across...grrrr