One million dollars is a lot of cash, but when it comes to retirement expenses, that money may not last quite as long as you think.

Retirees are living longer, and general living expenses are continuing to rise. And considering you may not be able to rely on Social Security benefits as much as previous generations, you'll likely have to lean on your savings for the bulk of your retirement income.

This means $1 million may not go as far as it used to in retirement. But just how long will it last?

Senior couple sitting on a bench at the beach

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It depends on your lifestyle

Obviously, how long $1 million will last in retirement largely depends on how much you're spending each year.

One common retirement rule of thumb is the 4% rule, which states that if you withdraw 4% of your total savings during the first year of retirement and then adjust your distributions each subsequent year for inflation, your money should last approximately 30 years.

Say you retire with $1 million in your retirement fund. If you want your savings to last 30 years, you'd be able to withdraw $40,000 during the first year of retirement, then adjust your withdrawals each year for inflation.

If you expect to spend far more than $40,000 per year, $1 million won't go as far. Usually, U.S. adults 55–75 expect to need more than $135,000 per year to enjoy retirement as comfortably as possible, according to a survey from Charles Schwab. At that rate, $1 million will last less than a decade.

Factors that can complicate things

It's also important to keep in mind that life rarely works out as neatly in reality as it does on paper. Even if you expect to spend $40,000 per year or less in retirement, that doesn't always guarantee $1 million will last 30 years.

The 4% rule assumes your spending levels will remain relatively consistent over the years, save for adjustments in inflation. But in reality, 80% of older adults experience significant spending shifts throughout retirement, according to a report from J.P. Morgan.

Healthcare expenses in particular can throw off your spending plans. Usually, retired couples can expect to spend nearly $300,000 on healthcare costs alone, a report from Fidelity Investments found, which can put a serious dent in your savings. Even if you do have money set aside just for healthcare costs, this expense can be unpredictable, and it's tough to know exactly how much you'll spend.

Preparing for the unpredictable

So how long can you expect $1 million to last if you're going to inevitably face spending shifts and unpredictable costs in retirement? There's no simple answer, but the best thing you can do is create a strategy that's as comprehensive as possible, then try to stick to it.

Run your information through a retirement calculator to see just how much you should aim to save, because $1 million won't be the right goal for everyone. Next, build an emergency fund with at least three to six months' worth of savings, which can help you avoid withdrawing more than you should from your retirement fund if you're hit with unexpected expenses.

Finally, once you do retire, try to stick to a budget to avoid burning through your savings faster than you anticipated. You likely won't spend the exact same amount year after year, but if you overspend one year, try to cut back the next year to make up for it.

Planning for retirement can be tough, and no one has all the answers. But the more thought you put into your strategy, the better off you'll be.