For a startup company looking to gain investor interest, which of the following seems like a reasonable amount of time to forecast ahead?

5 years is the reasonable amount of time.

Depends on the investment cycle: structured debt must be paid back in full; investment cycle shall not be cutted-off at low or high peak but at average normal stage because perpetuity will be biased if not; business must be cutted-off at steady cruise stage, otherwise present value will reflect an erroneous growing or lowering business concern. Generally, RE and Hospitality developments take no less than 15 years, even 20, 30 years to be fully completed, and financial modelling must reflect that. On the other side, many mature industrial businessess will be correctly pictured with 7-10 years projected. To my assistants I use to say that if nothing else matters than consider 10 years projections...

5 years-----------------------------------

5 years is a reasonable time

5 years, according to the company product sale and progress

it depends, but a 2 year forcast plan and another adjusted plan for another year shall give you a clear idea where are you heading in your adventure

! year is about expected as this ties in with budgets and commitments and forecasting. Investor interest in funding startup capital is not in the 5 year or 20 year timeframe

For any start up business as a standard it takes between 3-5 years of time to have clear forecast and have focus on it. the investor would surely look up the tenure and take a decision accordingly

Five financial years with achieved growth and 5 year forecasted growth plans.

I'd say 3 years detailed plan / forecast and stick in a brief description for the next 5 years. Use milestones and always have an exit plan, ALWAYS ! Investors will look for your plan of exit before anything else. They want to know what would you do if the business does not go as intended. 

Good luck. 

The  Forecast Period depends upon the Scope and Scale of the  Business and the Total Investment needed with clear demarcation as to the  CAPEX and OPEX with a  considerable period of  Investment required as Working Capital for getting stabilized the business processes and revenue earning cycles.  

As such a  minimum of  7 to 10 year forecast is necessary clearly showing the  CAPEX as stay put invested and  Opex as invested, recovered with profits and re-cycled as a  source of  working capital,  totally freeing up any further requirement of investments in the general business line; but only for any linked diversifications as a specialization or  added value of revenue generation.