It takes a couple of hours, if I download 100MB in the morning I normally see it come off sometime that afternoon.
You do get SMS notifications letting you know you're 50% and 80% through your bundle, then again when you have 4MB to go, and finally when you bundle is depleted.
We, at MTN get no notifications. Their counter does not work at all and we are in the dark as to where we are with our bundles. That is their way to get us to go over bundle and charge us excessive OOB fee for up to 3 months backdated.