R53, “too much” depends on the sum, your age, and rate of return.
When you reach are 72, the IRS will require every year that you take the 401k value at the beginning of the year, divide it by your life expectancy, and make a minimum withdrawal of that amount. They have a chart for life expectancy. To make an exaggerated example:
Suppose you are 50 with $1 million in your 401k, and you expect to make 7% per year. (At that rate, your principle doubles every decade. So by the time you are age 60, you’ll have $2 million, and by age 70, you’ll have $4 million. So, let’s say you have $4 million at age 72, and your life expectancy is 20 more years. $4 million divided by 20 is a required minimum withdrawal of $200,000. But, you’ll have earned $280,000 on the $4 million that year, and you’ll have to take out much of that, too. Let’s say $200,000 more, on average, per year as an estimate, for a total withdrawal of $400,000 per year.
This will be taxed at regular income tax rates. If you are single, and the rates don’t change from today, the rate will be at least 35%. Don’t forget that your social security income will be added into regular income, too.
If you are younger, you can have a smaller sum in your 401k to get the same result. If you are older, you’d need a larger 401k sum to get the same result.
In my particular case, until age 45 I was putting 10% of my salary into my 401k, and had, maybe $400,000 in there. I thought that was too much, so I reduced my contribution to 6%, because that got me the full company match, and that is free money that you should never pass up.
Off the top of my head, I’d estimate that $200,000 at age 30, $400,000 at age 40, and $500,000 at age 50 is “too much”. You should always do what is required to get the employer match, but not put more in the account above that. If you want to invest more, open up an after-tax brokerage account. The federal tax rate on gains held there, for over a year, is 0%-20%, so it’s not like it’s a terrible sacrifice.
If you’re on the border, create a spreadsheet and figure it out for yourself, or hire a tax guy who will do the math for you. There are articles on this but they can be hard to find.