It depends on your goal. World first? Top 10? Pre-nerf? Or clearing the raid just before the next content arrives? What degree of efficiency are you aiming for? 'cause all of the brackets have guilds with different mentalities in the way they are raiding in terms of the balance of quantitative and qualitative aspects.
If you want to kill all bosses at least pre-nerfed, it requires some ingame hours per week. Guilds which doing this achievement with relative low amounts of raiding days per week are quite rare. A lot of guilds are lying about their actual raid times, only advertising 2-3 days in their description, but non-communicated optional raiding days aren't unusual - and they are using those days quite a lot in the first couple of weeks and sometimes even further than that.
It is also an illusion to think a guild would clear anything if any member would just investing 2 raid days + 1-3 hours offraid per week. If a guild is successful, its the result of the few members which are investing a little bit more time in the game than the plebs which are going offline until the next raid.