How Long Does Unity Last? Installing Unity More Than Once.

How long does the free copy of Unity last? I vaguely remember reading in somewhere that it lasts 2 years

From the FAQ:
“How many machines can I activate my Unity license on?
Each EDUCATIONAL Unity license allows a single person to use Unity on one operating system that they have exclusive use of.”

Does this mean that after installing it on one OS, I wouldn’t be able to install it on another OS even if I don’t use the previous OS anymore? Lets say I’ve installed the one in 2012 on my first computer, then I format my computer, does this mean I can no longer install a new Unity anymore? or is it saying that I can’t have 2 or more Unity installed AT THE SAME TIME?

Each STANDARD Unity license allows a single person to use Unity on two operating systems that they have exclusive use of. FAQ