When is a good time to add someone as a facebook friend?

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What I'd say is go over and say a friendly hi next time you're round that department.
Being friendly in reality will aid your chances of her accepting your request.
Once you have greeted her at least a second time there is no reason you shouldn't add her on facebook.
Another thing to look out for is mutual friends - it's not a necessity to have any but if you do it's a confidence booster when adding people on facebook.

Really, the worst that can happen is a rejection, and that might not mean anything when it comes to reality.
Check if she has other co-workers on facebook as some people might not like to have the office haunt them after work

But mostly : go ahead and add her!