[cisco-voip] Upgrading CallManager 3.3(3)

[Adam]

I need to upgrade a CallManager cluster from 3.3(3) to 4.2(1). The
documentation says that I can only upgrade to 4.2(1) from 3.3(5). Is
this non negotiable, or is it possible from 3.3(3) as well. If so
approximately how long does it take for an upgrade of cluster with a
publisher and a single subscriber with less then 100 phones? I'm
trying to work out how many/long maintenance windows need to be
scheduled.