Whenever you feel ready and have enough money as well as spare airplanes.
You need four gates at the airport you want to base out of, plus a pretty good bit of money. I remember it was like $9 million to $16 million (or something around there) depending on the cost of the gates.
Last round I would order some starter planes for a new base, then when they arrived I would start up the new base. No sense having a base with no airplanes to fly out of it.
