vs protoss, try to get in the habit of grabbing gas around the 6 minute mark.
in silver league, do players use the FFE build as protoss? or is it mostly 4 gate?
if it's 4 gate, you want to get a 14 pool, and a 16 hatch, then a 17 extractor (though some players are skilled enough to not grab an extractor til later). creep spread and spine crawler count have a lot to do with the outcome, but as far as gas timing goes, you will almost always get speed faster in response to this, than you would vs a FFE toss.
anyway, the reason that you grab gas around 6 minutes (if you do a 3 hatchery opening vs a fast expanding protoss), is because you want to have your drone production completed before you take your gas- so that you can be economically stable enough so that you can spend all your larvae on units by the time they push out. you take gas at around 6 minutes, because that's when you should have enough drones to be stable enough to grab some tech/upgrades and even roaches if you want to.
people will expand on the toss builds more if you tell us specifically what matchup you need advice on. i just wante to give you a brief explanation about it before we hop into a discussion.
vs terran, i open with a fast expand (15 hatch) and grab a pool at 15 supply and gas at 17 supply. if you do a queen heavy opening, you want to get your 4 queens out before you take any gas - but if you like having early speed (like i do, which helps with getting you map control, harassment, while queen builds are more defensive in nature) you should use the 17 extractor build. it also allows you to start a +1 melee upgrade while you're droning, which gives you a head start before you fully engage in tech.
vs zerg, i like aggressive openings, because, you typically want to always keep their drone count lower than yours, so that you have a huge advantage over them in all phases of the early-mid game.
i open with 15 pool (to be safe), 16 hatch, and 17 extractor. i start speed right away, saturate my minerals, and take a baneling nest with my next 50 gas. next 100 gas i start a lair.
there are gasless builds that are becoming more and more common in ZvZ, but they require good queen micro, and good walling. you basically want to have all your timings 100% on spot, or you could get ling all inned before you even complete your wall-off.
anyway, what matchup did you want advice on in general? and then we can go into more depth on the gas timings :D
Edited by KILLorD on 1/27/2013 3:27 PM PST