It's rather obviously hard to give too much of an answer without some idea of what kind of trailer, how it is or is not secured, etc.
This Yahoo question and answer suggests it might start being a concern around 50 mph, particularly for tall 5th wheel trailers. (They also point out, quite reasonably, that having an awning unfurled on the windward side can assist the tip-over.)
These are wind speeds are where it's getting difficult to walk around, particularly if it's at all gusty.