Well, as low as you can get. You should still be able to be reasonably accurate just it will take longer for drift to show up.
The link below has a pretty decent write up on drift aligning, starting with the azimuth adjustment first where most people I have seen setting up seem to do the altitude adjustment first.
If you do the azimuth first per the link, then if you can't get down low near the horizon then at least you have gotten he azimuth right first so you will not have azimuth error turning up when you are trying to adjust the altitude.
http://www.astrophotography-tonight....ent-procedure/