I think Lieutenant to LtC should take maybe five hours, acting as an extended tutorial. Then LtC to Commander should take 20 hours, Commander to Captain 40, Captain to Rear Admiral LH 80, and all the rest after that remain 80.
This is based on the industry statistics that indicate the average $15/month subscriber plays 20-21 hours a week. It would give them the ability to keep ahead of the content curve for the AVERAGE player in the future (assuming the dev team really does get the promised expansion after the F2P conversion is complete), and allow continual work to be done in the mid levels where it can attract new players, instead of 100% of it having to happen on the high end.
The poopsockers would still get there in a week, which would keep them happy.