If you are looking a significant distance away from your current employer, look to combine a miniature vacation with your search/interview. You get the opportunity to check out the area and can write some of the expenses off on your taxes (at least in the US).
If you have decided to look for another position, you might as well start now. Unless a contract is holding you for the next six months, are there any benefits to staying? You might make an annual review with possible salary bump but why not figure that into the baseline salary for the position you are looking for? You need to consider costs of living in a new area as well.
When I have changed employer's, the most difficult decision was that on leaving. Once that choice was made, time frame really did not matter much.
Regards,