If you call your carrier, they should unlock it immediately if you are eligible.

I've been on plans in the past where I could unlock and where I could not (all prepaid). I switched to TMobile a bit over a year ago and bought an unlocked phone from the start, but my boyfriend stayed on Virgin Mobile. We realized that Tmobile will work better for him so he got the unlock code from VM yesterday, and we are going to experiment and see if he can bring his iPhone over, since the phone is still relatively new for him and he'd rather not buy a new phone yet. I'm not sure if his iPhone supports the right bands, but at least we can try now. (This was part of the reason why I left Virgin Mobile. They had it locked and so I could not even buy a foreign SIM when I travelled.)