How long we can use MSDN service after invited?
As long as you want.
You're pretty much wrong , he'd only have 3 years or less if your account is a bizspark msdn account which i can tell you myself since i have the same service amd even more from what you're promoting here
Correct is 3 years from the date you got accepted in bizpark, and if only 1 of your invites are abusing the account all the invites from this account will be closed