While it is hard to place a dollar amount on ROI for a piece of software that my company didn't pay for (I'm a VMware vExpert), I feel it helps me do my job more efficiently, which allows me to work on more than I would work on if I did not use it.

Instantly, for the Free Version.  

For the Enterprise Version, it probably would have been realized in about two to three months. This is estimated because my company never bought it for me. I bought it for myself, so I did not have to mess with the alternatives.

Not measured.