We have a four month old Cocker and we are not sure how far he needs to be walked.At moment he goes for a short walk in the morning then again at lunchtime,with the evening walk being around 20mins.These walks do not seem to be exhausting him but I am not sure that walking him further will not damage his bones.
Any help much appreciated.  

I'VE HEARD ABOUT 5 MINUTES PER MONTH OF AGE IS ABOUT RIGHT, SO 20 MINUTES SOUNDS PERFECT.

Until mine was 6 months old I concentrated on busy, rather than distances.  I thought it was important for my dog to see as much as possible in the early days so I made a mental note of the experiences I wanted him to have and I'd plan our outings around these.  We live in a small rural village so we could manage a short walk in the mornings to see farm animals, ducks, chickens, tractors and elderly people at the Post Office/village hall.  In the afternoon I'd drive somewhere where we could sit and watch heavier traffic, trains, buses etc and we'd do a short stroll past schools and shops for more people interaction.

Although the outings lasted longer in total I kept the actual walking to about 10-20 mins (depending on age and terrain).  We also did lead walking at training class and in the garden.  Somehow in this way my dog also never learnt to pull, which was great when he got bigger!

Don't forget to watch out for stairs, jumping on and off furniture and, in and out of the car - all can do a bit of damage if not controlled. HTH :)