I've never kept note before but I would say 3 to 4 months by the time the head moults out red, its the last thing to moult. Give em plenty of dark greens like dandelion leaves and broccoli etc and they will go red enough. You will need a sulfur drug to stop them going light through the moult, ive found baycox the best and they need a 2 days a month until they finish moulting. Feed them on a decent quality british finch mix.