It seems more distance than time related. With the larger back rest my wife should be able to do about 250 / 300 mile. She was OK with the seat but She found the stock back rest awful when cruising at 70+ for any length of time. Basically with the low screen and being higher than the rider she found herself being pushed against the top of the cissy bar rather than the pad. This caused major problems for her back a few days later. The convertible won't be as comfortable for her as the Heritage, where I had Gel filled touring seats and a large bucket back rest, but it should be OK.
As for the Red LED fuel gauge, I'm hoping to have it fitted on Friday. Failing that it will be a couple of weeks later as we're off to Croatia this weekend. Out of interest no one in Harley will confirm that it will actually work on the Convertible tank but also no one has come up with a reason why it won't work, so I'm quite hopeful. The local dealer has gone out of his way to get one for me. The part number is 70900040.