Nobody is going to be able to tell you how many hours is on your piston by simply looking at the top of it. The correct way to check your piston is with a micrometer and an inside bore gauge. You mic the piston at the skirt and compare the clearance with the bore. Your service manual will tell you the correct clearance. If you don't have a service manual, you should get one. If you don't have the proper tools to check your bore, look at the skirts of the piston. If they are excessivly worn and shiny and look like they may be rounded off a little, replace the piston. If those skirts break, the piston can turn inside the bore and cause some very expensive damage. If you have any question as to the time on your piston, it's better to replace it while you still can.