All right, some math time. Let's imagine 3 scenarios, a SS-oriented BLM (Best Case) a DRG (Average) and a barely-using any SS SMN (Worst case).
I simply took the default BiS builds from Ariyala's and the stat weight for this calculations. I used the simplified (Stat Weight)/2 formula for calculating DPS.
BLM:
514 Spell Speed at 0.258 Stat Weight.
Selene would add 154.2 extra SS, which in turns translate to around +39.78 DPS. At half uptime, this is +19.89 DPS.
DRG:
430 Skill Speed at 0.178 Stat Weight.
Selene would add 129 extra SS, which in turns translate to around +22.962 DPS. At half uptime, this is +11.481 DPS.
SMN:
410 SS at 0.0.94 Stat Weight.
Selene would add 123 SS, which turns into +11.56 DPS. At half uptime, this is +5.781 DPS.
So there you have it. Most of the time the real number should be somewhere around where the DRG stands. A BLM focusing on SS isn't very common and that is a very specific case, whereas a SMN ignoring SS altogether is the expected. Do remember that Tanks get a smaller boost, and SCH is on the same boat as SMN as far as DPS goes (So Selene doesn't add much to the SCH's own DPS).