With the caravan on, I prefer to have a comfort break every 1.5 - 2hrs or so, and don't like to spend more than 4-6 hours a day towing. Although I enjoy it, I find it can be more taxing than driving solo.
However, on a tour of Scotland a few years ago I ended up towing for much longer than I would want to again. We were in Grantown-on-Spey, site 4 out of a planned 5, when I started to feel ill. As the only one licenced to tow the caravan before the BE change I decided that we would leave the site and go home, instead of the planned 3 nights in Berwick-upon-Tweed, in case I became too sick to tow. The 450miles back to the Midlands took us over 12 hours and it was 1am when we got home. I was beyond tired, coming down with something, the weather was horrible, and we had to take numerous diversions as motorways started closing for maintenance during our journey. In the end I would have been fine to spend 3 days in Berwick as planned, and would have had a shorter final leg.