40 seconds for a regular plate filled with chips and a normal amount of cheese. then, after that 40 seconds, if it needs more, go for 5 to 10 seconds after.
Make a white sauce first then add the cheese. Doing that the cheese sauce will stay creamy and will not get chewy.
2 Tb flour
2 Tb butter
Melt this in a pan and stir until the flour and butter are combined
Add 1 Cup of milk
Cook and stir until the sauce has thickened
Add your cheese. for this amount a good hand full, or two.
Stir and remove from heat as it melts.
Add salt, pepper or a dash of dry mustard if you like.
Pour over chips or stir in some cooked pasta for an awesome Mac-n-Cheese
Thirty seconds in the microwave for the cheese. Dump on top of the chips and then just enough time back in the microwave to make sure everything is hot enough.
I put chips in the microwave and melt cheese on them all the time. No longer than it takes I have never had chewy or soggy chips from doing it in the microwave.