The phones come preactivated with $5 of airtime. You are probably safe to wait a couple of months to start using the phone, however, I would suggest you try it now just in case there is a problem with the phone or the phone number and you need to exchange/repair it. Note that when you start using the phone SO will start deducting 99c/month for 911 fee. Still, that is a small price to pay to be sure everything works OK.
In any case you will need to activate the $100 air time voucher within 90 days of purchase.