How long is the Demo Version of Mach3 good for?

Inivator

As long as you want, it has some limitations, 500 lines of code being one but there are quite a few others which escape me at the moment.
Hood

There si no limit to time. The only limit is that you can only use 500 lines of GCode in your programs.

So unless you want to rout-out sailing ships sailing on a choppy sea you will probably find it lasts a life time. I have been using it for over a year now, and never got near the 500 lines limit.