Will we receive notification when we have provided sufficient critiques to warrant posting our own material for critique? I looked for an answer to this specific question and could not find one, though I imagine it's been asked before. I simply don't want to post my own material, only to have it removed later.
No. However it stands to reason that should you post your material, and any questions are raised that a simple View All Posts search would reveal whether you have completed the required amount of reviews. So, just do 'em and post your stuff. In fact, do more than just two to be sure.
No, the main thing is you have to be here for 2 weeks, post 2 constructive crits, and have more than 20 posts total (not including word games). If you've done that (and as Faust said, why not more than 2 just to be sure?) you should be fine.
There is no notification. The constructive critique point requires the moderators evaluate your critiques. If they aren't up to par, your thread will be closed until you do have sufficient constructive critiques. If you havent met the more mechanical requirements, you simply won't be able to create the thread. It pays to read all the material regarding posting in the Writing Workshop, Most closed threads are the result of members rushing to post without taking the Writing Workshop instructions seriously.