You have to remember, what one country calls a fruit fly, another country calls a pommace or vinegar fly. The Drosphila we culture for live food are not the type of fruit fly, such as the Mediterranean fruit fly, that did so much damage in California to fruit still on the trees, and are terrible agricultural pests in many countries.
North Americans call pomace or vinegar flies fruit flies commonly but they only infest rotting fruit or veggies, not fruit on trees. Different species.
It is not hard to culture the flies. I culture wingless ones and those with stunted wings that don't work. Flies vary also by species. D. hydei has a month long life cycle, so it takes some time to get production going. They don't like it too cold or too hot.
D. melanogaster are smaller than hydei, half the size, and only two week life cycle. Maggots will pupate and hatch into flies in two weeks from the egg hatching into a maggot, or larva. A bit less fussy about temperature but cold will really slow them down too.
If you want a culture to continue producing, you do need to start new ones before the old ones die off. Get a suitable food, either commerical or find a recipe online. Or I can post mine. Add about twenty five or thirty flies to the new culture with a bit of netting or something for adult flies to land on and let them get working. This way you can keep them going for some time. If you have several jars on the go, if one becomes infested or moulded, you have others. If one crashes, you have others.
Melanogaster, I find, is just easier to raise than Hydei, but if you need a larger fly, Hydei is the one you want. They also come in flightless variants.