I've been playing for 4 years in game... that's... 4x12=48 days. And my DOP is around 70,000,000. The time when I create another base is when I have got saturated profit, from a place.
Giving an example. Shanghai Pudong, I have routes to Japan major cities, China major cities, South Korea, Taiwan ... and a few of Europes. I think that's enough for me. So I started Beijing. I think flights that are going to smalllll places are not so profitable, like going to Altay, Aksu... etc
Of course, you can play much more in Large cities (300,000 or 350,000) than medium/small cities.
It's you own choice.