Generally, no. If your engine is in relatively good shape, and your mower is stored indoors in a dry area, then you can get several years from a spark plug. (NGK plugs used in Honda engines have been known to die occasionally without warning, so you may want to replace them regularly, or keep a spare on hand.) Don't use the E3 or any other 'miracle' plugs. Regular Champion or Briggs & Stratton plugs just need to be inspected occasionally, and cleaned and gapped as necessary. The tip of the plug should be a light tan color with little build-up. If it is fluffy gray or black, your engine is running rich, but the plug can be cleaned and reused. If it is covered in oil or gas, it can be cleaned with carb cleaner and compressed air, but replacement might not be a bad idea. If the porcelain is cracked, replace it. I have pulled rusty, nasty old plugs from mowers that ran fine. Most people only use their mowers about 50 hours per year, so the plug should last a long, long time. If you are asking because the mower is not starting, the plug is probably not the problem. 99 times out of 100, the culprit is a clogged carburetor caused by untreated gas left in the mower for more than about 2 months. The newer the mower, the faster it clogs because the emissions carbs are more sensitive. One ounce of Sta-bil per gallon of fresh fuel every time you fill your clean gas can is the best thing you can do for your mower. Use two ounces per gallon the last few cuts of the season. Top off the tank before you put it away.
No, this will not work with 625 Briggs and Stratton.
It is definitely for L-head engines.
Yes, this will work on a Q6500 inverter.