Jorge,
For the Nelson engine, you can run either the Nelson standard glow plug or the heavy duty. If your engine is setup as received, you needn't change the plug after every run. The heavy duty plugs will last longer than the standard plugs but some claim the standard plugs are easier to "read" after a run. ie. easier to tell whether you should make changes to the needle setting or the head shims.

However, at this point in the interest of simplicity, just get yourself 6 or 8 heavy duty plugs and, once again, go run it. I routinely put 10 or more runs on one heavy duty plug before it needs changing. Again this depends on how hard you are on the engine in terms of the needling etc. If it glows after a run and it's not too distorted go ahead and run it again.

I am running .196 on the sleeve and about .013 on the head and I usually fly at locations 1800 to 3500 feet above sea level.

Randy S.