I think 3 round would be perfect. Another first-round only mock draft before then would be fun, too. We pulled it off yesterday with not too much trouble.
If you wanted to keep it in the same format for 3 rounds, without trades, I think you could pull it off, but if you wanted to get more advanced, like trading picks, I'd add a couple changes.
First, allowing people to trade picks would mean certain teams need to have designated GMs. The format of allowing anyone who post their pick first as what counts would make tradings picks more complicated. Maybe before the draft, you can ask people to volunteer to run two different teams, and the teams that go unfilled can be open to the public, with the first pick being it but no ability to trade. Or something like that. Teams with a GM can use .Net's PM service to discuss trades.
Another thing that would be cool to see, no matter what format you end up doing, is if you use an online Google Excel app which can be open to the public and is changed in real time. Mockingthedraft.com uses this format and it works great to track picks & trades and makes it easy for people to see which players were selected ahead of them and even top prospects still on the board.
I'll offer to help set up and run the Excel app if you need. PM me.
"People who don't punch their ponies in the face make me sick." - Louis C.K.