I can only speak from my personal experience, but I am on my 3rd phone from LG and Virgin mobile (also other android phones from other carries/phone brands) and I have NEVER had an OS upgrade on any of them.
So in my opinion, I wouldn't expect a Lollipop upgrade EVER.
If you want an OS upgrade, they expect you to buy a new phone. That's how they sell lower cost phones. Now when it comes to the high $ phones like Samsung - you may get a little better treatment. But, then again, you paid quite a bit more for that luxury.