You're looking for FG, not OG (OG= original gravity, FG = final gravity).
Depending on the starting gravity and health/quantity of yeast pitched, the beer could be finished in a matter of days. Still, yeast will continue to clean up fermentation byproducts for some time afterwards. I leave all my ales in primary for 2 weeks (longer for gravity > 1.070ish) as a matter of procedure. You could probably get away with as little as a week for a lower-gravity brew, but I encourage you not to rush if you can afford to leave it in primary longer.
You could use an autosiphon, but you should consider investing in a beer/wine thief. Just make sure everything is sanitized.
Definitely taste your sample (just don't pour it back, obviously). It will change a lot once it is bottled/carbonated/aged, but you should taste as much as you can at each point in the process. Also, if you pick up any diacetyl (butter/butterscotch) or acetaldehyde (green apple), you know the beer needs more time on the yeast cake to help clean those up.
Once it hits FG, it can generally use some more time to let the yeast clean up some fermentation byproducts. Yeast may be done fermenting sugars in as few as a couple of days, but they will continue to break down other byproducts for a little time afterwards.