Actually, it is a trick question. If your immediate response was to come up with a number (be it months or years) however close you are to an answer, it would only be partially correct.
Times have changed. The answer is simple – it really depends on the nature of your startup. Being involved in the startups in the web world, I can say that it is rare to have plans that are valid for more than one year.
However, the real point is – what can we do in the same one year (in parallel) while we are executing our plans?
The answer for that one is to build a structure within the organization that is agile and adaptable for the future years.
So, the real question should be:
“How quickly should you get your organization to be agile and nimble?”
and, the answer should be