It is very possible to get 5 of every class to L60 within the 90 days. I am fairly casual and got 17 toons to 60 in the 90 days. I never multiboxed before this and I quested/gifted them up. This is with only 3 RAF linked accounts to 2 master accounts. I am certain that I am on the low end of what has been accomplished as well.

Boosting with 4 linked accounts will probably be even faster.

Also, make sure to plan ahead, I didn't plan enough and made some "mistakes" with toons on certain accounts.

Good luck!