[scirocco]

You can use internet straight away, but the cost will be deducted from your credit balance at whatever the base data rate is. The cheap Internet Max rate isn't guaranteed to kick in immediately although in practice it may not take the full 48 hours.

And no, Internet Max is a per-monthly deal - you can't have two in the same month. The 500MB is not rigidly enforced by all accounts so unless you are tethering or streaming loads of videos you may be okay.