No one can tell you, apple don't typically pre-annouce releases.
However it is expected that there will be one soon. As it has (according to the
buyers guide of this site) been 213 days since the last update and the iMac has been updated, on average, every 205 days.
Alot of people, myself included, felt that it would be updated last Tuesday, yet it was not. You really can't tell.
I plan to wait for the next update, as I don't really need one until summer anyway, and considering the iMacs 10th anniversary is early august, plus a new iMac-suitable intel processor is coming out june/july, I expect it to be very probable an update will be made sometime between now and mid august.
I will probably buy whenever that is personally.
That said, if you need the machine now, its still a fantastic machine, its just for me my need isn't urgent so I'd rather wait for the next revision.
But no one can tell you for sure when they will be updated, could be tommrrow, could be not for another 2 years! (very very unlikely!)