Minimum length for a shed ramp 8" high using 2x12" PT?
5° is standard (ADA) which is 1:12. So at 8" height you would have an 8' ramp. If this is too long to line up the mower, shorten/steepen it as little as is necessary for you to use it. How steep is too steep is impossible to know without understanding all the circumstances. 

Find a solid sloped surface somewhere nearby and test drive the mower up and down, to get a feel for how steep is driveable and safe on your mower.