Usually by 8 weeks you can tell what the color will be - sometimes a little longer if its blue/aqua color.
By the time they are 5-6 months old it should be the adult color. RB kittens at 4 months may still have a bit of gold with the green - they usually don't get their full green eye color till 6 months old. If they have it good at 4 months - it will be much greener/deeper as an adult (good thing)
Charlie (5 1/2 months old) still has a gold/green eye color. Its listed as being gold on the registration form, so I don't know what it will end up with.