Most people apply to the airlines at the earliest opportunity, not because they necessarily want to fly bigger aircraft immediately but because you usually have to show/create a history of applications; a sort of waiting list if you like. If you wake up one day and decide yep, it's time to get an airline job well good luck to you, it might happen, but history has shown it usually takes a little longer than that. So you just have to decide/guestimate when you'll be fed up with instructing(if you will at all) and plan ahead accordingly. Hope that helps.