how do you refresh the ship dealer?

[jasta85]

I'm trying to get a specific model of ship and its not showing up in the ship dealer, been stopping by it in between pretty much every mission but still not showing up. I read that it refreshes over time but it seems you need to do something else, because I left the ship dealer, flew my ship back to home base and docked then went to do something else for an hour and then came back flew back to the dealer and only 1 ship had changed.

Do you have to do a certain number of missions or enter so many maps or something else aside from just let time pass?

[Pappy]

It's on a timer. Keep checking.

[Isidonis]

7 minutes - after such a time 1 ship changes in the inventory,
14 minutes -50% of ships changes in the inventory,
28 minutes - 75% of ships are chnaged,
56 minutes - changes entire inventory of a particular ship dealer.

[Misfit]

later on you get perk that makes it faster and much more, but this is a good bit into the story.
for the start you just have to wait.