We measured the depth of our favorite dining chair and went with that. It was 18 inches. The cushion overhangs it by about 1 inch and half to make it comfy on the knees. It is perfect for me at 5 4' and my DH who is 6 2'.

Here's a drawing that the lead restaurant designer at DH's office did for banquette/booth dimensions. 24" is the total bench depth. That depth accounts for the back cushion also.

Sarah Susanka's books have good information for planning the seat dimensions. Also, here might be some useful info here, esp. JM Seattle's sketch:

Here is a link that might be useful: Banquette seating-love it or hate it?

I agree about Sarah susanka's books. It is really important that you measure the banquette bench to be comfortable with nice cushions. If you're doing 3" or 4" cushions, and back cousins, the depth is quite a bit more but the height is quite a bit lower as the cushions increase height.. My architect designed it so that the seat has a 1" slope towards the back and the back has a 3" angle and a lip under the window sill for the back cushion to sit into. I also decided to have a deeper window sill and a deeper seat as I anticipate that the window seat will be used for lounging purposes where the sitter is not always facing the table. I am thinking some extra movable cushions will make this comfortable for when you are eating as well. The edge of the window seat should have about a 2-3" lip to allow people to tuck their feet under who they sit. Without this, they will keep kicking and banging up whatever base you have.