It's acumulative (2 days of preparation means 176 Sorcery Points into Spells slots), he can prepare a destruction day and wreack havoc.
Etherealness can be used to infiltrate a fortness.
But some of those spells (the Extended ones) will be be finished before two days, so you'll have to recast them. The table I have provided took into account what was necessary to have the spell you mentionned active during the day, supposing you started at your maximum of 20 sorcery points. The ability you had on the day before to take short rest couldn't get your more SP, since it is assumed you start at your maximum.
Only the day you cast everything, your remaining slots are : 0/0/3/3/3/1/0/0/0. You have the time in the day to still take 11 short rests, 5 to get your SP back to 20 and use thoses to regain slots (at a price of 2 points for a 1st level slot up to 4 (your max slots) so 8 points, and 3 points for your 3 2nd-level slots (so 9 points), bringing you down to 3. Then you could could get back to 19 using 4 SR or 20 using 5 SR.
So, in a day where you start at your maximum and want to end up attacking something at the end of the day, you'd be at max spell slot (up to level 5), depleted spell slots for higher level as shown above (1/0/0/0) have 20 SP but... you'd have to do 10 SR after your LR. That's not a lot of adventuring time, as 62,5% of the time you'd be a sitting target. I'd advise anyone to avoid attacking your when you're at peak but rather during one of your plentiful SR. (There is a smarter way to prepare for a big day, but it will be your job to find it).
Please provide the regular assortment of spell cast to last the whole day on a regular day, so we can see what ther Sorc capabilities will be on a regular day, when it is probable NCEST will be attacked.
(PS: you really shouldn't attack a fortress by yourself since it would expose you to Legend Lore by making you legendsworthy).