I think it depends on what kind of blog you're writing and how long your posts are.
Mine is an informational site, and I tend to write longer posts. I never post more than once daily. I usually post 5 days a week - Mon-Fri. Then on the weekends I try not to even THINK about my blog. :)
If you're talking a 1-2 paragraph review or a quick link to something else, I think 2 a day would be okay. I'd consider putting the links up on my sidebar also, AND constantly reminding your readers about them in the next week or two's worth of posts. For example, at the beginning or end of your post, say something about, "And don't forget to check out this ongoing contest at such and such site if you haven't already..." and put the link there.