I can tell you how it worked for us, as we are staying 14 days in December & I've just been booking my ADRs.
The CMs on the phone will tell you that it is their policy to only book you for the first 10 days after your check-in. For the remaining 4 days she told me that I had to wait until the 90 day mark to book those.
However, when I used the website to book ADRs online, I was able to book 4-14th the first day my 90 days opened. The next day I could book my 11th day, the next day my 12th, etc. I had no problems with it at all, and as soon as I linked my resort reservation it opened up those days.
The problem I ran into was calling in to book Fantasmic and Candlelight Processional dinner packages. I had to call 3 times until I got them booked, but did in the end. They might tell you they are not able to book past 10 days from check-in. I called back, and the 3rd CM said "yep, it opens up for the 17th, it shouldn't, but it does, so we'll go ahead and book it".
All that to say - you should be able to book the next few days after your 90 days, but you also might run into some CMs who won't do it. Just call back and try again.
Good luck!
Andi