Time horizon is the total number of years your portfolio must sustain withdrawals - typically from retirement age to life expectancy. Longer horizons increase exposure to sequence-of-returns risk and require more conservative withdrawal rates.
Time horizon is the planned duration of retirement - the number of years between when you stop working and when your portfolio no longer needs to fund expenses. It is one of the most consequential inputs in retirement planning because it determines how long your money must last and how much risk the portfolio absorbs along the way.
How It Works
Time horizon directly maps to the simulation length in a Monte Carlo simulation. A retiree at 65 planning to age 95 has a 30-year horizon; a FIRE practitioner retiring at 45 planning to 90 has a 45-year horizon.
The relationship between horizon and risk is non-linear:
The longer the horizon, the lower the safe starting rate: the 4% rule was built for 30 years, and each extra decade asks the same savings to last through more bad markets and more inflation.
Each additional 5 years of retirement lowers the safe withdrawal rate - the same savings have to be spread over more years.
Why It Matters for Retirement Planning
Time horizon interacts with nearly every other retirement variable:
- Sequence-of-returns risk: longer horizons create more opportunities for devastating early losses. A 40-year retiree faces twice the sequence risk exposure of a 20-year retiree.
- Longevity risk: underestimating your time horizon is a common planning mistake. Planning to average life expectancy means a 50% chance of outliving the plan.
- Inflation risk: 3% annual inflation is manageable over 15 years (about 64% of purchasing power left) but severe over 35 years (about 36% left).
- Asset allocation: longer horizons generally justify higher equity exposure, since stocks outperform bonds over extended periods - but only if the retiree can tolerate the volatility.
A Practical Example
Two retirees with identical 1,000,000 portfolios and 40,000/year withdrawals:
Same savings, same 4% withdrawal - but Retiree B needs the money to last 15 years longer, through more market cycles and more inflation, so the plan is far more likely to run short. Running both in a simulator shows exactly how much.
How Retirement Lab Addresses This
Retirement Lab derives the time horizon from your retirement age and the age your plan runs to, then simulates the full period (1,000 futures on the free plan, 50,000 on Pro). Change the end age to see how each additional 5 years changes your success rate and depletion risk - making the longevity trade-off visible and quantifiable.
Frequently Asked Questions
- How long should my retirement time horizon be?
- Financial planners typically recommend planning to age 90-95, not average life expectancy. A 65-year-old couple has roughly a 50% chance that at least one partner lives past 90. Planning to age 85 means a coin-flip chance of outliving your money.
- How does time horizon affect withdrawal rates?
- Longer horizons require lower withdrawal rates. The classic 4% rule was designed for a 30-year horizon. For a 40-year horizon (retiring at 55), a lower starting rate is safer. For a 20-year horizon (retiring at 75), you can usually withdraw 5% or more.
- Should I plan for a fixed time horizon or use life expectancy tables?
- Neither is ideal in isolation. Fixed horizons risk being too short or too long. A better approach is to run Monte Carlo simulations across multiple horizons (25, 30, 35, 40 years) to see how sensitive your plan is to longevity - then use the longest plausible horizon for planning.