Leslee, the program (engine) does not 'expire', you can use it forever.
But the virus database updates (VPS) and the program updates expire in 14 months. After that, you should register again, for free again, but that is the policy here. 
what timing I was just about to make this same post today
I knew I would be checking my avast forum to see how this wonderful product is holding up against this new SP2 (2 as in "2 confusing")
OK I see you reply to how there is only 14 months of free vps updates \and I need to register -again- but you could do me and I think all a favor by x-plaining this a bit deeper
If I put my registration number in won't it recognize me as already having a trial key ?
as well I put in my e-mail addy and also got cut off for "already registered"
Please don't say we have to uninstall and start again
Pleaseeeeee Technical say it isn't' :ot soooooo .............?
mine expires in 2 weeks