Optimal bcrypt work factor

What would be an ideal bcrypt work factor for password hashing.

If I use a factor of 10, it takes approx .1s to hash a password on my laptop. If we end up with a very busy site, that turns into a good deal of work just checking people's passwords.

Perhaps it would be better to use a work factor of 7, reducing the total password hash work to about .01s per laptop-login?

How do you decide the tradeoff between brute force safety and operational cost?