This dimension is the most forgiving when you are working on a backless bench, but is a bit more finite when you’re designing a chair.
Rule of thumb is 15 to 20 inches. I typically like to go at least 18 for most applications, chairs and benches alike. Now, you may need to go beyond this measurement if the angle of the back of the chair extends beyond the 10- to 15-degree range typical of many chairs.
I especially like to add more depth for outdoor chairs. They are designed for much more relaxed situations – people tend to slump a bit more in outdoor chairs. The more slump, the more depth required. Try it sometime and you’ll see how much farther up the back of your leg the edge of the seat rests.
Also apply the above to benches with backs.
Backless benches allow you a lot of flexibility with depth. A wider bench is often more inviting with its ample room, plus it allows those sitting to face in opposite directions – a bit antisocial in some respect, but a key comfort point. An example is my Party Bench plan – which I designed to allow for comfortable seating by many folks in a party setting.