The answer is - as so often: It depends
In our group we normally start by 9 or 10 am and are done around 10 to 11 pm - including two larger breaks for food. Last time we had - IIRC - 17.000 points (not PL) per side BUT we divided the battlefield into two sections. On one side I fought to defend the imperial city with 7k points of Custodes, Ultramarines and Imperial Knights vs. 2x 3.5k points worth Tyranids and Orks. On the other side we had two players also defending the city with 2x 2.5 points of Imperial stuff vs. 3x 3.333 points of Chaos, Necrons and Mechanicum. (Yeah, I know - the lore doesn't fit but that are the armies we have...). So we had essential two different parts but kept the turns on sync.
What I'm trying to say is: If everyone fights everyone all over the battlefield the match gets really, really slow. You constantly have to wait for someone so you can shoot at him or to shoot at you. What we are trying to do is simply "divide and conquer". Everyone gets a certain role in a more or less defined area. So he fights only 1-2 people at the same time and they mostly fight against that player. So we reduce the waiting time a lot and can manage to get mostly 4-5 turns out of the day (instead of barely 3 in the past). Yes, you can argue that it is fun to fight all over the battlefield and getting involved with everyone but let me tell you: That does not work well with an apocalypse sized battle.
I wouldn't go to strictly into a fixed time slot for the turns if it is just a fun game with buddies. The target should be the fun part of the game and that is killing stuff. Don't make a complicate mission. Just put your armies on the table and beat the :cuss out of each other
As for table size: We tend to build everything centered around a larger imperial city. This will included about all of the points of the imperials and has some cover on it. It is a standard table size maybe including a couple of inches from the surrounding landscape. As for the attackers, they have 2-3 standard table sizes around the imperial city. Sometimes they have to march along the long edges, sometimes the short. Take into account, that you can only reach so far as a human. So having a large square of 3x3 tables makes it impossible to play on the center since you can't reach there. Make a Z or L or T shape with the tables. Makes it a lot easier.