Yes it is quite long time. But i think it is not only readying servers for the patch itself.
WoW did the weekly maintenance, now, wow continue to reboot the servers each week. Sometime, for some servers we can see the 5-10 classical minutes to reboot are not enough to go back on the servers... have to way like 30-45 minutes. So they do speed maintenance on specific servers.
FFXIV get servers which runs more than 7 days. even if they dont drink coffee waiting the next maintenance, they cant do all the servers need for years of work.
End of my speculation so : they uses those big patch maintenance to add / remove components (to, later adds instance for pyros or anything that would need this for minor patches, or even this major patch). Or simply do a big maintenance on all server.
Personnally i prefer a weekly maintenance with good timings (from 3 AM to 8-9 AM for example) but this system get a big matter : some people due to their work plays the morning (even early). I can perfectly understand the "big maintenance" FFXIV see, not what i prefer but also another good way to do it.
For the 24 hours, i think they dont need all those hours most of time. But better be sure to do all the work done in time you did announce to your customers, than being short, and unexpected issue makes you being late... really late. WoW did also have those "patch day no play" but was the bad way to have it (heh, patch will end at 1PM... oh no finally wait two more hours, and then 6 PM, not possible to log in... ... rhaaaaaaaa)
Yes, we will maybe have like so much previous patches other matters in servers the tuesday because all people will be on same spot, because they are unable to do a patch that sread people enough (or marshmallow server maybe). Yes we will probably have an emergency maintenance on thursday. YES it is illogical to do 24h maintenance to correct issues 2 days after. But maybe all i said earlier is false, and i am not engeneer or server technician.