This will depend on what a 'serious' trail means to you and what you expect to be able to do on it. My son rode over 100 miles of very serious trails (mountains, water, mud) on an 80cc Yamaha Raptor. He only weighs 100 pounds though. If you're not looking to go fast, you can probably do a lot. Other factors like suspension and ground clearance are going to determine what you can do as well. I would ask someone who's ridden the trail you're contemplating.

SP