I would allow
2 hours to get there! It is about 36 miles, so to get there in 35-40 you would literally have to be flying and not hit any traffic at all, which would be impossible.
The Grove is very nice and is a great place to spend the day! There are other stores and great restaurants! I think they may even have a cafe inside the store? I have not been so don't quote me on that.
Like I said, I have never been to the store but maybe 60-90 minutes in the store? Obviously more if you do any of the activities they offer
http://www.americangirl.com/stores/events_index.php?location=LA
Hope this helps!!