I see from the messages on this thread that it varies. My RIR is 28 weeks & doesn't even have a comb/wattles yet, while others have posted theirs lay as early as 20 weeks! So I guess the easy answer is... anytime between now and next June??? LOL!
Not having fully developed the comb/wattles may (or may not) be related a lower amount of protein in the birds diet. Protein is the building blocks of ALL tissues in the body, including the skin, which the comb is. This is why I have chosen to feed starter feed to my girls for life (20-24% protein) instead of grower or layer feed which is typically 16-18% protein levels. In growing birds the higher protein will help in all areas of growth, maturing them more completely and possibly a bit sooner. This is also done by many BYC'ers with good success. Chicks can be raised on nothing other than starter feed thru the entire life cycle if desired, with no ill affects. I have also heard that high protein helps to shorten molt times as the birds replace feathers. You may try it and find no difference, but I am sold on its benefits. Good luck!