Gripped vs Non-Gripped Nikon DSLR

Hi there, I am building m'y second camera trap and I am looking for some information that I can't find anywhere !
How much standy time do you get from a Nikon en-el15 batterie assuming all your settings are put up to drain the battery the less.
The longest I've put up a camera trap without changing batteries was 3 weeks. But if I remember correctly there was still life left in my batterie.
I' m assuming the the life of a gripped camera on stand by would be doubled with a grip but id like to know if its worth buying a grip or not.
Thats why I'd like to know the average stand by time if a Nikon dslr with a en- el15.
Thank you

I have found that the camera battery will easily out last the flash batteries.

Thanks Jeff