It is possible to do this tour in 4hours (for a fit and experienced hiker). A normal walker need on average 6-7hours.
There is a short part, about 50vertical meters, where it necessary to climb a bit.
After the lake Bessvatnet everything else is a easy tour.
Keep in mind thats a tour with more than 10km length and more than 1200m cumulative elevation gain.
there is also a path next to the lake where you can return to Gjendesheim:
I recommend to avoid weekends and very sunny days, it will be very crowded.