We actually ran a closed beta program for a month or so with an invite system. We released invites slowly while we got the product up to speed, and then launched.

And, yes — there's more to going fulltime than hitting X customers. How much cash do you need in the bank to avoid cash flow issues? What's your margin like — is it really X users, or is it X*2? What's your churn rate? If you don't know the avg. lifetime of a customer, you may be out in the cold.

We had a closed beta of one month. We kicked it off on HN about 6 weeks ago:

http://news.ycombinator.com/item?id=2941589

Couldn't have done it without HN!