How long can the Razor MX500 Dirt Rocket be ridden after charged?

Run time depends on a few things, including the weight of the rider, the terrain being riden over and the condition of the bike.
For example, a light rider riding on flat hard packed dirt or paved road with a bike that is in good mechanical condition - tires inflated to correct pressure, chain correctly adjusted/lubed, brakes adjusted, etc. - will have a longer run time than a heavier rider on soft undulating dirt with a bike that has not been maintained.
Generally you should be able get 40-45 minutes out of a new battery.
Check out the trouble shooting guide and user manual if you are getting less than half and hour.
http://www.razorama.com/ownersmanuals.html