You can follow one of two stock-based strategies:
1. Invest 100% in an S&P500 index fund, returning 8%/year [1] and growing to $11M in 30 years [2], minus withdrawals.
2. Invest 100% in bonds, or a high yield savings account, yielding 0.6%/year [3] and growing to $1.2M in 30 years [2], minus withdrawals.
You want to do as much of (1) as possible, while doing (2) as little as possible in order to pay for things (mortgage, rent, food, kids, etc.). You can't rely on (1) for day to day expenses, since stock returns are super variable (recessions, depressions are inevitable over time).
To balance the two, you can think of it as: consider retirement when MIN(1, 2) > expenses. The exact mix of 1 and 2 depends on your personal risk tolerance.
EDIT: Numbers for (2) were off, as suggested by multiple people in the comments. Updated.
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[1] https://www.investopedia.com/ask/answers/042415/what-average...
[2] Calculate this with an online interest calculator (being sure to re-invest earnings), or remember the rule of 7 (your stocks will double in value every seven years).
[3] https://www.bankrate.com/banking/savings/best-high-yield-int...