The only server it is 15 min is Express.

The duration to market is scaled approximately with the server's 1 turn every X minutes.

For example,
Primary is 1 turn every 30 minutes --> 4-6 hours for goods to market (8-12 turns).
Alliance is 1 turn every 20 minutes --> 4-5 hours for goods to market (12-15 turns).
Team is 1 turn every 15 minutes --> 2-3 hours for goods to market (8-12 turns).
Express is 1 turn every 4 minutes --> 0.25-0.75 hours for goods to market (3.75-11.25 turns).