Went from 19 - 45 on my Monk in about an hour. Seems like all you have to do is leave the zone in some capacity and come back. As long as you aren't in a party during this process it resets. As a Monk, this makes things really easy with the Zen Pilgrimage ability. All you need to do is use the ability twice and you're teleported back to the graveyard of the corresponding invasion at ~0% Phase 1. You can also just ride a flightpoint out to the nearest zone and back, but for Monks this is cake.
Assuming the XP rate is steady as you level I could see making it to 100 in a day or so. I just don't have the time to test it all the way up.