If you create a character you can use the USERS command to get an idea of how many characters are around during any given hour, along with some other interesting stats.

The max online on any given day has hovered between 25 and 30 for most of the year.

45 |                                                                        |45
40 |________________________________________________________________________|40
35 |                                                                        |35
30 |                                                                        |30
25 |   ******                                                               |25
20 |***************   ******                                 ***      ******|20
15 |******************************                        *********   ******|15
10 |************************************************   *********************|10
5  |************************************************************************|5
Time 15 16 17 18 19 20 21 22 23 0  1  2  3  4  5  6  7  8  9  10 11 12 13 14

So "prime time" would be 15-20 CST, or late afternoon through mid-evening.