The first trip i did with my daughter was to the US at 6 months, longest i think is either London to Hawaii via LAX at 15 months or London to Hong Kong via Doha at 20 months
Not depending on a stroller can be a good thing, we didn't get ours returned during two long layovers in Doha (first was 6 hours, second was 4) and managed fine, i've only travelled alone with my daughter and have managed fine on several long haul trips