This is a route which starts in Oxford (on the Isis) and finishes in Henley (although could also be done the other way around too). It's a pretty long and I managed to do it in one day as part of a sponsored row in an 8 but it was pretty exhausting and you'll need a good supply of tape for blisters!
If you're after doing it at a leisurely pace or have smaller boat (i.e. slower!) you could do it in 2 days or even 3. The route is really lovely and you'll become an expert at opening and closing locks by the end of it too.
This is a beautiful stretch of the Thames to row and can be rowed in sections. The clubs along the way are welcoming and friendly and can always pointers and help. Would make a great challenge rowing the length of the Thames I've done it on my bike so maybe now time to row it.