This is really a question about the production side of the show...
How flexible really is the format of BB? Regarding things like nominations and themes like annihilation, is that decided well in advance, or do they just make it up as they go along?
A lot of people believe the whole thing is fixed, to the point that production selects the winner and then skews the whole programme for that person with editing and influence from presenters and SOTS.
I'm not a conspiracy theorist myself, but I find myself wondering whether the original plan for this series meant to have this many HMs in this close to the end.
And speaking of the end, the announcement about it finishing 2 weeks sooner than expected also seemed to come overnight, and nobody anticipated it.
Sometimes it seems as if the whole show is at the whims of a select few at the top and it can do a 180 at any minute.

I think it's far more fluid that it used to be. If it gets dull, or they are all getting on too well then a plot twist is unveiled.

I'm sure they have a general outline of when to introduce things, but when it fux up - a la the other hosue, they change it up.

The producers have done well with Annihilation week. Getting rid of some of the horrendous cool kids.

Unfortunately they've given Jackson far, far too easy a ride.

On the evidence of recent series, about two and a half minutes.

They have to have a plan but allow for flexibility in the event of unseen events, such as a hm walking or being removed. Also things like the place being flooded as has happened this year. They have to be able to think on their feet and adapt and be able to respond to unfolding events but all the main points (last weeks theme and the other house at the start (the duration i mean) etc) will have been planned inadvance before the show even started.

I suspect they make it up as they go along!

The 'others' was a complete disaster, fell flat on its face & nothing came from it. All that actually happened was they had a houseful of HM's who didn't have to fight for their place in the main house! The other house which was built appears to have been a white elephant.
Their ploy with the exes was another fiasco which has come to nothing!

They decided for reasons best known to themselves that they would wind up 2 weeks early. No reason given at all, made no sense, as this has, despite everything, been a really good BB, the best for a long time actually.

They certainly didn't envisage that the favourite (lateysha) would have been voted out by a HM, nor did they envisage that Ryan would have been kicked out (again by HM's).

The producers must all be running around like headless chickens wondering where it all went wrong (for them anyway).

I saw the thread title and immediately thought "about five minutes", but you're probably closer

I genuinely believe that they only plan the first week, everything after that they make up as they go along.

Annihilation week only happened because the series was shortened. They made it up so they could get people out quickly.

Apart from the first week or two (and they're lucky if the first goes according to plan) I would say seconds, if that. Perhaps it's simpler to say they make it up on the spot now, even during live shows with heavily-promoted twists.