Hi just a few questions
Is the pro license just for updates from current 3.0 until 4.0, then you re buy? or is it pro license forever?
How often do new versions of Unity get released is it every single year?
When was the last release?
Thank you
It will be a pro license forever but you only get free updates for 3.x
You will have to buy an upgrade to get 4.x updates (likely, going by the past).
Unity 3.1 just happened recently and Unity 3 was just released 2 months ago 
When was 2.0 released?
in 2008
Unity Version cycles in the past were in the range of 18-24 months