There are several ways of drying the grapevine by baking. Some people use a higher heat for a shorter amount of time while others prefer a lower heat and a longer time. I will generally bake mine for several hours depending on how thick the pieces are at 250 degrees.
Any wood, not just grape...is really a guessing game that only experience teaches well. Things that make a difference: how thick, how many layers high do you have it stacked, how wet was the wood when harvested...and even then, it's not an exact science. I HATE drying grape, in all of my experience it is probably THE hardest to get perfectly dry this is why I dry, dry and dry some more...probably baking mine much longer than anyone else. I wouldn't personally bake at over 250 degrees. I boil mine first so the baking isn't to kill...it's to dry. I dry at a lower temp for much MUCH longer times.