You'll need to keep your dog away from newly seeded areas of the lawn at least until the seeds germinate and the grass begins to sprout. Germination can take anywhere from a week to a month depending on the grass type and site conditions.  If you're only reseeding a small patch of grass to repair the damage, you can cover the spot to keep your dog from damaging the seeds. However, if you're reseeding the whole lawn at once, you'll have to keep your dog out of the yard altogether, which might be difficult.