League-of-legends – When do you start buying wards, and when do you stop

Buying oracles elixirs, like many other things, varies intensely by which champion you're playing!

In general, Carries should not purchase Oracle's elixers until they have most (if not all) of their core items. They squish much too easily to make it a worthwhile investment.

On the other hand, tanks and supports are the best candidates for "Oracle" duty. Supports make use of the elixir because they're expected to roam the map while placing their own wards. Any wards they find when placing their own is gold in their pockets and sinks their opponents'.

Tanks benefit from oracles because they have the survivability to protect the buff from fading with a careless death. Since tanks usually are also the initiators, Oracles allows them to keep the battlefield free from tactical stealth in response to their initiation (such as Akali Shroud or Talon Ult).

The circumstances change a little bit if you're playing against Shaco, Teemo or Twitch, as you'll want to trend towards more pink wards / oracles across the entire game. Without the advantage of their stealth and surprise, these champions will quickly fall to your team.

In a game to game basis, there really isn't much you can do to get your team to buy wards other then just asking and hoping, but when you get to the higher elo games people take it seriously and will also be buying wards. In lower elo games/unranked people don't take it as seriously/have the same skill level as high elo games and either don't buy wards or take offence to someone calling them out on it, which just makes the team do worse. If you're the only one buying wards, just do your best to get wards around dragon (although this is less important in unranked/low elo play), baron, and your lane.