Its very difficult to answer. Its also depends on how many friends you have made, the best tags you have used. So can't guess exactly.
I don't think there's any answer for this question. I guess it's quite random and depends on the tags that you use, keywords and also time of the day the post is submitted.
I never use Stumbleupon. No traffic to my sites so far. I also don't like it's homepage interface. It's so difficult to list a website page.
The better the post you submit is the more visitors you'll get. You might get a few extras if you have more fans but you'll only get fans by recommending good sites, I don't think it's worth trying to use it like this tbh
I also just registered myself recently. But not so sure about this stumbling works. but will stumble upon the sites i like too Please stumble my site if you like it too, thanks