90 days after your last top up, the service will be suspended and you'll have 29 days to make another payment and restart your service, after 30 days however, you'll loose your number and boosters and will need to buy a new sim.
If you're not on prepaid and you get a monthly bill:
You have two options: you can change your plan to seasonal hold. For 10$ per month you can change to the seasonal hold plan. This plan allows you to keep your service active but not your phone. When you come back to you just change plans back to an available plan. The drawback to doing this is that if the plan you are on now isn't available when you get back, you won't be able to switch to it again. So you have to decide if it's worth it to potentially loose your plan or just continue to pay your normal bill.