Poisoning after grapes or raisins is unpredictable. Not every dog develops kidney damage, but serious cases are well documented and there is no simple amount that can be considered safe for every individual. Research has pointed to tartaric acid as a likely toxic component, which also helps to explain poisonings after certain other products that contain a lot of tartaric acid.
Early symptoms can include vomiting, diarrhoea, lethargy and reduced appetite. If kidney damage develops, urine production can fall and the dog can become much more ill. The sooner the vet gets information about the intake, the better the chance of judging what needs to be done before kidney damage has had time to develop.
Contact a vet right away and give your dog's weight, the approximate amount, whether it was fresh grapes or raisins and when your dog ate them. Do not wait for symptoms, and do not make your dog vomit at home without instructions.