Hi, you will find many topics on here dedicated to similar discussions. There is no definitive answer. Generally at next overhaul. The railways were/are a business and taking anything out of service would mean lost income so they wouldn't do it any earlier than was required. Additionally the railways only had workshops appropriate to the levels of construction/maintenance they needed to support their business. They wouldn't have had the capacity to take on many additional repaints.
I can't go to my books ATM to see when the southern corporate livery was set but you can expect the ballance to shift more towards Southern corporate livery the further you get from 1923. You'd also find pre grouping liveries looking more tired as time progresses.
Rule 1 can be stretched quite a bit to suit what you want to model.