CyberCubed
Yeah, ok!
I think she will stick around for about 1~5 seasons, depending on how long the diamond/pearl anime is.
Seasons are really only for the dub, the Japanese version goes by sagas.
Unless the writers REALLY stretch out Shinou and make it as long and drawn out as boring Johto, it's safe to say they'll be another short Orange Island/Battle Frontier type saga after Shinou where Hikari will remain for.
I would imagine Hikari will be here for at LEAST two sagas, (Shinou and whatever the "short" saga is after Shinou), like May was here for Hoenn and Battle Frontier.
Essentially though, she'll be here for the entire D/P series. I wonder what they'll do when the "third" game comes out, after all, AG wasn't called "Ruby/Sapphire" but instead it was called Advanced Generation. They're going to have problems with the title once Shinou ends.