A normally fit walker can do it in two days, roughly four hours each day.
It's all on very easy paths, no need to worry about heights. The initial climb from Rütli to Seelisberg is steepish, and there is a lot of upping and downing on the second day (Flüelen to Brunnen), but on the whole it's very straightforward. Be warned that there's quite a lot of road walking, especially between Isleten and Flüelen, then between Flüelen and Sisikon.
I did it a couple of years ago as part of a complete walking tour of the Vierwaldstättersee, see http://isitmuchfurther.blogspot.ch/2...e-stage-1.html
for a description. The complete tour of the lake could just about be done in a week (I did it in ten short days).