Legal speed limits aside... you should only go as fast as your personal limits, and your trailer and tow vehicle will allow while maintaining
complete control.
Depending on how well the vehicle combination is set up with regard to weight distribution front to back, and side to side, how level the combination is, how much tongue weight you have in relation to total trailer weight, how good your tires are, how much air they have in them, and a number of other important considerations makes the difference between being able to tow safely only up to 45 mph, or being able to run the speed limit plus.
A properly set up combination, with weights and driver experience considered, can safely travel at 75+ mph.
That said, an experienced driver will know when wind, traffic, and terrain will safely allow such speeds,
and when to slow down.
Like anything else, towing has a pretty big learning curve.
A lot of people with very little experience tow, and a lot of them go too fast.
Most times things go okay with only minor scares...
With practice and understanding of what can go wrong, and why, towing at the speed limit and beyond can be safe and enjoyable.
Rob
Waiting for "someday" will leave you on your deathbed wondering why you didn't just rearrange your priorities and enjoy the time you had, instead of waiting for a "better" time to come along...