Ten Years
I've heard it said that we over estimate what we can do in a year and under estimate what we can do in ten years. Working in the start-up community as I have over 10+ years I would have to say that this is true for most young entrepreneurs. I hear stories that "(place name of hot new start up here)" is going to raise $(place unholy amount of start-up capital here) and gain (place amount of new users/followers/monthly revenue here) within (place short timeframe here - typically under 18 months).
Why is it that we're in a rush to hit it big? Good logic and sense tells us that success is cultivated over time. VC firms understand it will take a good 7-10 years to see a return on their investment. Maybe we're in a rush because we see overnight sensations literally everywhere on the internet. In Hollywood they say it takes ten years to become an overnight success (again with the ten year theme).
I would argue that we're all grossly underestimating what we can accomplish in ten years. Most of us don't have a ten year plan, let alone a good plan for what we need to accomplish this week.
Regardless of what it is that you're setting out to accomplish, ask yourself if you're thinking long-term enough. If not, why not? Chances are you're not working on something that will keep you inspired and motivated over the long-term. Whatever the case, it's worth considering. Better to plan now for the next ten years vs looking back on the last ten years and kicking yourself for not planning.