How do you determine when your chain is beyond the manufacturer's wear limit? First of all let's correct the myth that chains stretch. Chains don't stretch. The outside measurement of an individual link plate does not get longer with wear. Your bike is not "so powerful" that is stretches chain. Chains are constructed of essentially link plates, pins, bushings and rollers. In order for a chain to roll around a sprocket the pins must have clearance through the bushing holes, allowing them to pivot. That clearance begins at about .0005", or 1/6 of a human hair. The pins and bushings are subjected to continual pivoting, and as they pass a sprocket, steel against steel wear occurs. This causes the pins and their mating holes to wear in an eccentric manner as the diagram below shows. In abrasive dirt conditions and especially mud, grains of sand are introduced between these mating surfaces and wear is accelerated many times over. A sealed chain (O-ring, X-ring, Z-ring ...) acts to prevent abrasives from entering this pivoting joint.
For all full sized dirt bike, the standard chain pitch is 5/8". Chain code numbers indicate the pitch. a 4xx chain has a 4/8 or 1/2" pitch. A 5XX chain has a 5/8" pitch. Sorry to those fond of metric units. Like pipe sizes the world around, chain is measured by the "kings thumb". 5/8" is the center-to center distance between pins. Most chain manufacturers limit chain wear to approximately .006" per link. So if you take that number and multiply it by 100 links, the accumulated clearance that some refer to as "stretch", is close to a full pitch or 5/8". So if 99 spaces fit in the distance that 100 spaces were suppose to measure, your chain is shot. So here are two ways to inspect your chain for wear:
Break the master link and stretch your chain out on a work bench in a straight line. Now count out 100 spaces and mark the first and last pins. Now take a tape measurer and measure the distance, center-to-center between pin 1 and pin 101 (It take 101 pins to create 100 spaces. Just like it take two pins to create one space.). If the distance is greater than 63-1/8", then it's time to replace your chain. A brand new chain will be 62.5". Don't get confused by the "tape measure" in the diagram. I like to "shave an inch" on the starting end and measure starting at 1", rather than trust the accuracy of the clip on the end of the tape.
The second method is not as precise, but will save you time in breaking your master link. Suck a wrench between your chain and rear sprocket by rotating your rear wheel by hand, until the top of the chain, along the top of your swing arm is tight. Using the method above, count out 24 intervals, and measure the distance center-to-center between the first and last pin. A new chain will be 15" dead nuts. If you measure more than 15-3/32" it's time to get a new chain.