"It's a good size, honest."
(None of this is really helping much, is it?
I don't think we're told exactly how long it is between the secret meeting at HQ and the launch to rendezvous with Chronos One. Transit to Chronos One was no more than a few days, probably no more than a day. They were at the rendezvous for a few hours, and then the torpedo struck the Chancellor's ship. From the arrest of Kirk and McCoy until their trial, we don't know exactly how much time passed, but I would think no more than a few days, based on the fact that they were *still in the same clothes* and the Klingons had not noticed and removed the patch on Kirk's back. (This and the idea that Uhura would not know passing Klingon are the weakest points of this best of all of the Star Trek movies, in my opinion.) From there, we know that it is a week between the end of the trial (and the rescue briefing in the President's office) and the conference on Khitomer. And the assassination attempt occurs during the President's opening remarks at the conference.
So, somewhere between an estimated 2 months 14 days, and 4 months 14 days, with the biggest factor in that 2 months being the time between Kirk being notified at the secret meeting and the actual launch of the Enterprise to get there. (Negotiating the terms of the meeting, scheduling, rescheduling, etc - but given that these are Klingons and the Chancellor would have been eager to proceed while political will remained behind him, I would say that 2 months might actually be exaggerating things.)
Now, my question: why?
(Just realized - you asked for time between the secret meeting and the conference. Subtract the original two months between Praxis exploding and the conference from my estimates above. I still had that time included, trying to provide total in-movie time for the whole film.)