Well... the decarboxilation (heating the weed in the microwave during some 2-3 minutes) will produce some smell... dont know how much time it lasts, as it depends on the ventilation of your kitchen... if you let the microwave open, open all the windows of the kitchen and let the wind enter, probably after 10 minutes the smell will be gone.
But if there were no wind, or if your kitchen had bad ventilation, the smell will remain for a longer time... maybe as long as 30 minutes or even more...

After microwaving the weed, grind it and put in the sandwich. Heating it until the cheese melt wont produce much smell, as the time of heating will be much smaller.

Anyway... if you want be sure to wash all the smell from the microwave, after making the sandwich prepare another food in it, preferibly some food with a strong smell... so it will cover pretty much the smell of weed.

And remember to eat the sandwich with an empty stomach. Its VERY important, cause eating weed with a full stomach doesnt work, and just wastes all the weed.