I've read in user guides about setting up 3G USB port for failover. All seems straightforward enough - WAN1 goes down, switches to 3G connection on WAN2/WAN3 per model.

However, what delay is involved in switching over?
Does the internet connection in use automatically switch back to WAN1 when it comes back up and if so what criteria are used to decide, e.g. does it need to be back up for X seconds before reverting?
Does the router just poll WAN1 to see if it's back and switch?

If 3G is setup as a backup, does any data go over it at all, i.e. would any usage be shown on the 3G connection and hence generate charges from the mobile supplier, if WAN1 stays up?

Many thanks in advance.