My question is about playing support in competitive environments. It's extremely common (indeed, standard) to see a support start with a Faerie Charm and some Sight Wards so that they can later build to Philosopher's Stone. It's also not uncommon to see Heart of Gold. Recently, I've even been getting Kage's Lucky Pick. One can even imagine a situation where Avarice Blade would be viable on a support like Blitzcrank (if you're intending to go more AD support than a full on aura items build).
How many gp10 items is "too many"? What I mean by this is: at what point does it become counter productive to keep getting items for the gp10? At what point are you getting worse returns than if you just went ahead and got that Aegis of the Legion or Zeke's Herald or whatever?
EDIT: By competitive play, I mean someone who is doing ranked games seriously in order to improve their Elo and hit 1900+, rather than someone who is just messing around in a Normal building AP Sona. This is someone who is genuinely serious about understanding the meta game in depth and wants to improve his skill at the game.
Best Answer
[Updated for Season 3]
The game is currently in flux (season 3 has only been out for a couple days), but with the boosted ambient gold gain, it appears you only need one gp/10. I've been building Philo Stone to great success. If you expect to get some significant assist-gold (you're confident in your kill lane, or you're Soraka and have a good eye for your ultimate), you could probably stand to buy a Kage's Lucky Pick, but I much prefer to only buy the one Philo Stone and start immediately after into my build (Sightstone, Shurelya's, Aegis -> Bulwark).
Again though, this is all pretty fresh. Your mileage may vary.
[NOTE: The answer below was provided for season 2. There were significant changes implemented for Season 3, and this answer is now out of date. It is provided here for history, but should not be followed.]
It depends. The rule of thumb is two (Philo Stone and Heart of Gold), and 98.355% of the games, you'll want those. If the game is going to absolute crap, teamfights started early and your team is getting routed, you may have to skip the second and start pouring that extra money into wards and an oracle's (counter to what you'd think, oracle's is a great purchase if you're falling behind and the laning phase is over).
I'd be very suspicious of my support if they got a third gp/5. The only real time I can see that being worthwhile is if you know it'll be a late, late game, and that you'll need to be a significant contributor in teamfights for damage.