The Prosperity Meter is filled slowly as blocks are mined. For every 1 block mined, the Prosperity Bar increases by 1.
The speed at which blocks are mined varies, however, so the actual amount of time it takes for the Prosperity Meter to be fully filled is not always the same. The Prosperity Meter requires 1700 blocks to be fully filled.
If we assume that the average block time at that time period is 12 seconds, then it would take roughly 340 minutes, or 5 hours and 40 minutes for the Prosperity Meter to fill.