Varies per site. If a site is known to be updated more often, the spider will come more often. As with all things, there are many factors which make up how often a site is spidered, but in my experience, frequent updates will increase the spiders visits.
I don't know exact but it may get Google to spider your site more completely and frequently as it needs to know what the content is on your pages so it may serve up the right ads. But it won't give you a boost in any way in the rankings. Won't give you any extra PR either.