At the very least I see him as a rotational player unless he gets some type of long-term injury. Now that he's shown that he can use his height and strength to his advantage, it would not surprise me in the least to see him included in some packages that would overload certain defenses with something that exploits their vulnerability against, say, height and speed as one example I wouldn't want to the be DC that has shorter and/or slower DBs to work with, then see a package come on the field that has some combination of Jordy, Janis, Davis and Cook lined-up as receivers and Cobb or Montgomery coming out of the backfield. That's a lot of poison to swallow.
I'm not as pessimistic as some that he could never become a starter or first off the bench type of a player. We just don't know yet, but we've seen some flashy returns on a mere 7th round investment. I once felt the same way about Driver and he turned out okay, and then some. And that did not happen overnight. Janis has the basic physical skills to at least have a shot at a high ceiling as a WR, that's for sure.
He's already a terrific gunner, and you can't help but notice that he defeats double-teams routinely with his strength and then often just blows past the guys trying to block him. Impressive.