It may only take once, or one grape/raisin, or it could be the 3rd or 4th or 5th time or raisin, or, it could take 20. The Vet. I talked to at work about it said there is no rythme or reason and no way to test or predict wich dogs it will hit harder.
Incidentally, I was alarmed as I read the book Holistic Guide for a Healthy Dog
that it listed raisins as a fruit to feed your dog in a homemade diet
There are so many people out there that have no idea how lucky they have been time and time again