Length really doesn't matter.
Second what is sig spam? Is there a sticky that has a in depth answer like less than 10 words, posts within 5mins, 100 posts a day, 300 a week etc
A tendency to make posts in order to boost your post count, and you have an ad in your signature, particularly one that pays you per post. There is no secret invisible line that you shouldn't cross in terms of numbers, it's mostly subjective.