It varies greatly among my sites, both the ones I have now and the ones I had in the past, but I haven't noticed a correlation between average visit length and the conversion rate or returning visitors percentage.
The nature of content dictates visit length. If you own an article-based site, with articles 500+ words in length, anything under 02:00 would be a poor result. On the other hand, if the website just provides simple information and quickly channels the visitor towards the operator (e.g. no deposit bonuses list), visit length and bounce rate are irrelevant as you do want the visitor to leave quickly.
For article-based sites, here's a quick tip for increasing the visit length. People only skim through the title and maybe the first two sentences in an article, so you got to write a good lede to hook the visitor and make him read the rest of the article. It doesn't matter how good or bad your article is as a whole, the title and the lede make all the difference.
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