No idea how long it'll take for a 5 man team to level to 60... so many variables

As far as getting ready, if you use DynamicBoxer for Isboxer/innerspace you can start multiboxing the day/hour classic opens even if you don't have characters created yet (you do need to make your team size placeholders and setup your windows layout though; and if you start on retail, you'll need to change or clone the slots to use the classic wow profile (basically change _retail_ to _classic_ in the paths))