Errr ... regarding that. Unless that was changed at some point Illya dies I think 1-3 years after the events of Fate / Stay Night even if she survives as they messed her body up too much adjusting her to the Grail. Granted I don't see this mentioned in the wiki when I checked it might not be canon anymore. I wouldn't mind if they did, the Girl deserves a Happy end dammit!
It's left intentionally vague. The only actual confirmation is Zouken stating she won't survive a year after FSN, meanwhile Ilya just parrots that information. But since then, all other evidence points to her living at least a bit longer than that, and Zouken was making an estimate of something unprecedented. Einzbern homunculus are said to be near immortal after all, under normal circumstances. He could also have been plain lying. We'll never know, since Nasu will probably never say.
Speaking of Tiger Dojo, the biggest hint to the contrary is when she states she would be around to ensure Shirou doesn't turn into EMIYA. That's where it gets a little contentious, where I think fans are too eager to play the tragedy card and insist she's speaking from the afterlife or something.
Seeing as Illya dying or being killed is a key reason for Shirou to either become EMIYA or take similar futile routes towards his dream, that implies she doesn't die prematurely. Plus Taiga is there too, so how is Tiger Dojo an afterlife? I don't want to get too serious about Tiger Dojo though.
The HF route also implies that there exists at least some ways to extend her lifespan. We just never find out if they do anything about it. Then there's other details or implications in Ataraxia...Apocrpyha...FGO, etc.
But coming all the way back to the original point. I said she becomes a lolibaba more as a meta joke, not literally. Whether she lives to a ripe old age or dies and watches everything from Tiger Dojo, she's probably still going to end up spending decades, if not longer, as a loli. Meanwhile, in FGO, Sitonai/Illya was deliberately emphasized to be older in the artwork notes, and she has at least two ancient goddesses in her, but she's still a loli, and younger than the original Sitonai at that. Now this Prisma Illya has been here for.....well goodness knows how long really. And she's still a loli.
