do you mean physically or mentally?
Most hit their full adult height between 9 months and 1 year, then fill out after that.Cockers aren't really considered physically mature till around 2 years, give or take, depending on the pedigree.
Martin Deeleys book is excellent, though it does address all flushing spaniels, I don't know of any that are just working cockers. You may also be interested in the old (pre-1950's) books on cockers, they delve a lot on the workers, as they were the origin of the breed. E-Bay is a great place to look.