1. You are entitled to one free update to the final version (RTM, GA, whatever) on your Windows 10 system. This system will update on or after July 29 if you have your Microsoft Account connected to it.
2. You are entitled to one free update to the final version (RTM, GA, whatever) on your Windows 7 system. You have one year from July 29 to update this system.
Microsoft has said that upgrading from Windows 7 or 8.1 will be free, and is yours to use and enjoy for the supported life of that computer. Those are Microsoft's terms for the free update. Take it or leave it. It is up to you whether you wish to take advantage of the offer.
After your Windows 10 system has been updated to RTM, GA, whatever, you have the option to continue testing as an Insider or stop testing. Again, it's up to you to continue testing or to stop. I would guess that if you stop testing, the same terms as those for an upgrade from Windows 7 or 8.1 will apply; you're good for the supported life of that computer.
Edit: BTW, if it were me, I would put the OS(es) I want to upgrade to Windows 10 on their final destination computer. That way, when July 29 comes, you'll be good to go. Of course, this won't apply to OEM systems; those are locked into the hardware.
This is my plan for when 10 goes RTM. On my Desktop I'm going to upgrade my Windows 8.1.1 to Windows 10 RTM for my main OS and dual boot into the test version of Win 10. My wife's laptop is going to be upgraded to Windows 10. Right now she's using Windows 10 TP. I'm still not sure what I'm going to do with my laptops and tablet.
If my experience is anything to go by duel booting two M$ O/S,s using the same key on the same computer is a no go.
Having had installed W8.1pro and W10 on two separate SSD,s the last update on 10130 converted NTFS on my W10 disk to RAW.
Not to mention my other drive with W8.1 pro needed a Scannow and a Dism before it could become fully operational again.
Yay, thanks MS .
I wonder if we'll be able to jump in/out of the Tech Preview, like if there's a broken ass build where the only thing I wanna do is submit feedback for the annoying bug and then revert to Final?
The last time I dual booted 8.1 and 10, 10 did something to my 8.1 drive. I had the Windows 8.1 drive unplugged when installing 10. Then plugged it back in, booted to 10 then to 8.1. 8.1 then reported errors or something and had to do a repair. It did eventually boot up, but I always wondered what Windows 10 did to it?