It made me laugh when I saw the press release from GP2 saying that he was helping them with their development. No he's not, he's using it as an outdoor gym to get fit and improve his coordination & focus.
Times are likely to be irrelevant, the car will be set up to give handling charachteristics that he wants to test, such as fuelled to the brim for stamina, or light as possible to generate max speed and check the lateral loads on his neck. He may also be doing a lot of pit stop practise, and possibly learning to share data with Jock Clear.
Oh and it gives Merc a bit more PR, more than Jensen ever could.