Go find one and sit in it. Same for the office chairs. Although if you look, especially at places like OfficeMax etc that'll have a good selection, they'll be labeled in classes. Some are good for upto 2hrs straight sitting, some upto as much as 12hrs. But it's your ash that's gotta park in it so it had better be comfortable.
Me, I hate my wife's ergo chair. It's one that's good for 12hrs. If I'm not seriously actively sucking down coffee, I'll be asleep in about an hour or less, it's that comfortable. $1000 chair, full lumbar support, padded wrist rests, thigh and hip support, massage, heat etc...