I'm organising my first tourny (yay!

My question is this - I set up the tourny on Tuesday (March 27th), how long should I leave it open for people to join? Want to get the balance right between getting as many entries as possible but not leaving it open too long that people drift away.
It will be a relatively short knockout tourny with a maxium of 5 games if you get to the final.
Any thoughts?
Thanks, Dave.