He absorbs magic from petricite, no matter how long ago the magic was cast. A dude couldve spat magic into a pebble of petrecite 4 billion years ago and sylas could harness it.
Please note: Galio is MADE of petricite, and is millions of years old, and has protected Demacia from devistating magic time and time again.
All sylas has to do is sensually stroke Galio and he could honestly take down all of runeterra.
We don't know if there's a limit to what Petricite (or Sylas, for that matter) can hold. My guess is there is, because you can use magic in Demacia -- it's just weakened or sometimes absorbed, which means that it can't really grab everything.
Also, I think if Sylas tried to, say, pull all of Illaoi's magic or all of Aurelion Sol's he'd likely just burst into flame.
I don't think that Galio has that much magic stored within him, he seems to use it up when he moves, hence why he stops moving after a while once no one is using magic around him anymore.
I really don't think Sylas can just absorb any magic that was cast during any period of time, especially not 4 billions years ago. Keep in mind, the Kayle and Morgana fight wasn't that long ago, only about 1000 or so years ago, after the rune wars ended and Demacia was just starting off as a kingdom. Plus, there's also the amount of power that was used during the fight, Kayle and Morgana basically destroyed a whole small city with their destructive magic, they both went all out. So no, I doubt Sylas can absorb a small dust of magic that was made an hour beforehand, let alone millenia.
EDIT: I just noticed that you specified petricite so, apologies on my part. But anyway, that's not particularly useful since petricite is unique to Demacia, while the walls can be used as weapons as they were in the lux comic, anywhere else, unless the place is innately magical or there's a mage around, his powers aren't that useful. Also galio isn't millions of years old, neither is demacia, hell godlike beings like Ornn aren't even millions, and I doubt Galio would just sit there and let it happen, once he wakes up, Sylas is pretty screwed.
If petricite absorbs magic and stores it and then Sylas gets that and uses against say Galio or in his area, wouldn't he just absorb it again?
Thank god that Kayle will smite him.
Jax, Syndra, AURELION SOL, Targonian aspects, Darkin, etc.
Lots of people higher up the tier list than Megaman of Magic.
I think that magic power that Galio absorbed is used to keep him alive and moving, so there aren't really any reserves in him. There could be some in other pieces of petricite, however. Also, isn't petricite just some sort of wood/stone that was affected by magic in Rune wars to become the magic-absorbing material? Its not really that old and op then.
Yo when Sylas finds 
's secret stash of world runes whats gonna happen?
Unfortunately for that theory, 
exists.
Sylas is still mortal...
We don't know if magic stays in petricite forever or if it drains out. And I seriously doubt Sylas can store ridiculous amounts of magic in his relatively small chains.
