Hard to say, Supergran! Surprisingly minute differences in sun exposure, soil, watering, etc. can make a big difference in ripening times. Add to that the tiny differences in genetics from seed to seed in a variety, and the differences in when they were pollinated, and there is no telling when the green one will start to ripen. If it is a mutant, it may even stay green, though that is very rare. There's still weeks of time.