I think that one week is a good time to consider crossposting. If you do so, please add links in both directions so that people who read your question at either place will have a chance to know everything posted at the other place. You can add a comment to the older post if you do not want to bump the question on the top page.
Since the FAQ entry about crossposting links here, I would like to express my opinion about something other than what is asked in this question, namely about crossposting in a short period of time (such as one day). I feel that it is unfair that some people crosspost a question in more than one place, usually MathOverflow and cstheory.stackexchange.com. Of course everyone wants to get his/her question answered as quickly as possible, and exposing the question in many places increases the chance. But they should refrain from doing that, like the rest of us.
Some new users seem to crosspost questions because they do not know where the best place to ask is. I do not think that it is the right reason for crossposting, either. For them, I would recommend the following:
- Read each of the candidate websites a little so that you can make an educated guess about where the best place for your question is.
- Post your question at only one website.
- If you do not get a satisfactory answer in one week or so, crosspost the question on another website if you want to. Do not forget to link in both directions!