You don't "specify a duration" for the valuation: an infinite duration is baked in the valuation itself through the time value of money. If the annual interest rate is 10%, 100$ in 1 year is worth 90$ today, 100$ in 10 years is worth about 35$ today and so on. Of course, in the case of company valuations, you have to also account for the uncertainty (both on the upside and downside) of the future cash flows.