You can calculate that using a youtube onboard video of a full lap and a stopwatch. If you want to be more precise you can use a PC simulation.
There are 6 heavy braking points at La Sarthe useful for regenerative braking: before Dunlop chicane (T2), before 1st and 2nd chicanes at Hunaudières (aka Mulsanne straight), before Mulsanne corner, Indianapolis corner, and Ford chicane before the main straight. Possibly Porsche curves too, but I'm not sure how would KERS perform handling-wise while braking deep into the curve.
http://fr.wikipedia.org/wiki/Fichier:Ci ... ck_map.svg