Workplace Culture

» mark7000 - is two week's notice enough?

I'm an owner of small retail & design store, (two other employees beside myself). One of the full-time employees has told me that they are applying for other jobs and that once they find a job, they will then give me two week's notice. The problem is, they could find a job next week, a month from now, 2 months, etc.
I like this employee very much and he has said that the desire to move on is a financial one only (mostly that they need health insurance benefits that we cannot provide). However, because of the nature of my business (design and home furnishings), it will be very difficult to find someone to replace him with only two weeks notice, and I also can't run the place without that spot filled. So I feel like I need to start looking now to replace this person. But, without really knowing when the current employee will be leaving. . .
I don't want to find a replacement and then have to kick the current employee out. It's kind of sticky.
I guess my main question is, should I be at the mercy of the current employee and wait until they find another job and give me two week's notice or should we just set a date (say, 30 days out) when they will leave (new job in hand or not), so I can aggressively start the replacement search now?
Thanks for any ideas.

-- posted by mark7000