How long can i have a contract phone before i have to activate?

Ok, so I want a Iphone 5c, they are.99 on contract with At&t. I am currently on gophone plan with a Nokia Lumia 635 lol. Ok, so can I order the iPhone and not activate it right away or will I be billed, or does billing not occur until activation? Basically I want to know how long I can have the phone after receiving it without activating until I get enough money to pay $60 monthly. Also, would I be able to activate it by putting my gophone sim card in it?

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