I remember reading somewhere that 12 blocks (as in 8 squares you can zone on) was the farthest away that a Cim will be willing to walk, from player testing. I doubt that's optimal in a lot of city designs, and I'm betting you will want much more frequent stops for transit to ensure people are willing to walk to transit and then use transit, then simply get in the car.
I try to keep a series of "feeder" transit lines close to a lot of people's houses (within three blocks) to heavily reduce the amount of people not using transit, and then some major routes to get people to other feeder transit that branches out into the rest of the city. Seems to have worked best so far at encouraging use -- more people on the feeder means more people use the rest of the transit system, I guess.
My original city had a hundred subway stops. That seemed to work too. Maybe?
A lot of it is just going to have to be trial and error. More than a few times I've had to eyeball problem areas and edit pre-existing routes to make it work better.
