i'm assuming you're riding a fixed, b/c you're saying "cog" singularly, and this entire thread has that FG vibe...

That being said, if this is your only bike, and you ride every day, that cog won't last a couple foo years. It's hard to say how long it will last; there are many factors, including the conditions under which you ride, the front:rear gear ratio, you're riding style, how far you ride each day, chainline, how clean you keep your drivetrain components, and more. Buy a chainwhip, or the other FG kids will never extend you any respect. Buy a chain breaker, too. a 15 mm wrench. A full set of allens. A tube of park or other decent grease. The most popular chain lube in your area. A spare chain. Tire levers. A quality floor pump. And, find a friend with a headset press, or make an "FU-2" from stuff you buy at the hardware store.

hth
=rob