As much as possible! You often hear the recommended 10%, but I think it's better to ask your grandson what his goals are. Is there anything specific that he needs (or wants, but I am more about needs)? If so, he should figure out what that goal will cost, and estimate from there how much he should save. Ideally, if he can take the amount needed for his goal, plus a little extra (just so some $$$ is left over when it's time to pay for that item), that would be great.