Greenlight by itself is pretty much useless for gaining an audience or getting new press attention. If you have the option of skipping it and getting onto steam directly, there's no harm done in not having a greenlight campaign.
Most of your organic greenlight audience will appear within the first few days of you submitting your game. If you don't get enough votes during this time, then you're in trouble, because organic traffic disappears. After the first few days, you will need external traffic source (e.g. press coverage, advertising) to drive viewers to the greenlight page.
Steam usually approves a new greenlight batch about once a month. There initially was a very large queue/backlog, but not so much these days. If you've got a decent game (one that can get significant positive votes in it's first few days), you'll usually get approved in the next batch -- so about 2-6 weeks. If you're unlucky you might sit in greenlight hell for a year or more.
In any case, I'd plan to do a PR push at the same time as the greenlight launch, so that hopefully you get some press, which will hopefully drive more traffic to your greenlight page to ensure success. I'd probably engage a game-specific PR consultant to go over your greenlight page itself and review your PR plan before publishing it too.
Most of your organic greenlight audience will appear within the first few days of you submitting your game. If you don't get enough votes during this time, then you're in trouble, because organic traffic disappears. After the first few days, you will need external traffic source (e.g. press coverage, advertising) to drive viewers to the greenlight page.
P.S. there's a lot of very dodgy services out there who promise to get your game through greenlight... Some even call themselves "publishers" and will try to get you to sign contracts where they own a large chunk of your income, when in fact they're just small-time key-giveaway twitter account. Some of these services will actually ruin your chances of ever being greenlit, because if Steam finds out that people have been bribed into upvoting your game in exchange for a chance to win free game keys, they might just ban you outright.
