Got a spare phone and put a pay ad u go sim in just wondering how long a number will last if I don't use the phone ?

don't know but I found an old orange payg phone that hadn't been used in 3 or 4 years. The credit had expired but it still took an incoming call on its number.

I head it was 3 months what I need it for is emergency calls the phone will be in tbe glove box of my car !

It can vary from network to network though in most cases it's about 6 months. Making or receiving an SMS or a call resets the clock