First of all, there might be mistakes in my post.
It means the same like raiding times on pvp servers, as those are limited the same way.
So in case of launch, I bet there will be lots of people trying to log in on at the first few hours, trying to avoid that purge as long as possible, building up a protected base, even in t2/t3 purge areas. (Newbie river being t1, north+volcano being t3, rest (including swungle) t2.)
That being said, this time frame depends on what setting has been chosen. I thought they meant they set these 18-22 for official servers, but maybe it became a default setting.
I kind of hope for some PvE-Mayhem servers - featuring 24/7 purges. 
Those 24 purges per day are the number of purges for the whole servers.
In case of 200 active players on that server each day, it basically means you got a <10% chance of being hit by purge during that time - given that your activities filled up your purgemeter.
→
You not having gotten purged yet means all the other days 24 other players have been purged.
(Though I guess there is the possibility of one player getting purged multiple times, but I dont know if that thought is true.)
So the time frame does matter, but the number of purges is way too low!
Also, for the question of who gets purged - all those who got a sufficient meter enter that to-be-purged-raffle.
I would prefer if the number of purges was changed to the number of unique players which have been online the day before. Also I think that bigger clans should be preferred by purge.
This 120 thingy setting decides on how often those points will be added.
In the latest devstream they presented a nordheimer purge while explaining a few things about purge.
This also included how those points will be gathered.
Basically farming resources wont give a number of points but only 1 point in case of gathering stone. Even if there were 20 people in clan who all are farming stone, only that one point gets added.
So every 2 hours (as 120 minutes are 2 hours) a clan can get 1 point for gathering stone.
That being said, someone else in some other topic told about building giving most points, followed by crafting.
I am not sure about that, but I think that setting was meant to allow up to that purge level.
As we have been told about 1,2 purges around newbie river, 5,6 purges in north+volcano and 3,4 purges on the rest of map.
The thing I am uncertain about when speaking of purge is:
How will official servers handle that minimum 1 purge per week on each player, when on a single day more players log in than purge can cover in a whole week? (That point is reached with the 169th player logging in.)
