They close the on-site car park for half an hour or so (depending on the crowd size) to allow the mass of pedestrians to exit safely. Obviously by this time, most people are back in their car and the wait ends up being more like 45 minutes.
If you're not in a rush to get back, you could always have a drink in the fans bar afterwards or if you are leave 10 minutes early and exit before they close it down.
Failing that, the closest off-site car parking can be found adjacent the stadium at Barton Airport, I think its the same charge of £5 as the stadium.
Best place to park however is Eccles RLFC, however this is a 15 minute walk to the stadium but you will get away straight away.