just saw this. i actually leveled last night and randomly chose this. unlimited or once a day?
@Dausuul is correct. Once per target per day, effectively, but an unlimited number of different targets.
There were a decent number of combats where I locked down the big bad with this power, and then just sat there whispering to them while the rest of the group beat down the fodder.
I'd note the details on the power carefully. To initiate it
you need to see a foe within 5', and it needs to be able to
see or hear you. Then, if you want to continue it, the creature needs to be able to
see or hear you and you need to stay within 5.
It is
not a spell. This has a few impacts. DMs may rule differently, but under RAW, a PC under the effects of a 2nd level invisibility can use this ability without losing their invisibility as it is neither an attack, nor a spell.
It also requires no spell components. I ran a wizard/druid for a brief bit that had this ability and used it secretly in wildshape to great and subtle effect (however, a DM may rule that you can't do this as you may need to have a voice - talk to your DM - but the purring cat that climbed in laps and mesmerized people was a lot of fun).
If you're hidden, a DM may rule that you do not need to leave hiding when you use this ability. In my games where I played the enchantment wizard (with boots of flying from a low level), I flew invisibly over foes, hidden, and then whispered in their ears to lock them down ... and the DM ruled I was revealed from hiding to the creature I was hypnotizing, but generally not to others.