My own observation from acting as a moderator in a couple of places over the years is that the first post of an account is almost always sufficient to tell whet

My own observation from acting as a moderator in a couple of places over the years is that the first post of an account is almost always sufficient to tell whether an account is a spammer or not. I would say that a better approach would be to moderate users’ first posts—or perhaps to do spam analysis and moderate any that are not overwhelmingly ham. But even without that, requiring fifteen messages before allowing links is going well beyond what could be argued as helping with spam protection.