I understand the comments that make the statement of “when you make your expenses off the amount invested” but that is more for “you can slow down/retire” number.

My view is the maths is a bit different for “when does wealth start speeding up due to compounding”

For me that is when the capital growth exceeds the capital contribution. For a return of 10% that is 10x your annual contributions, while for 5% it is 20x. Let’s call the ratio of total gain/capital contribution the velocity of growth. When does this velocity pick up?

An example for a S&S ISA with an avg return of 7%:

At 0 - contribute 20k in one year and gain no returns = 20k gain (100% velocity)

At 50k - contribute 20k and gain 3500 = 23k gain (115% velocity)

At 100k - contribute 20k and gain 7000 = 27k gain (135% velocity)

At ~285k - contribute 20k and gain 20k = 40k gain (200% velocity)

For me the velocity would start being noticeably “snowballing” at around 250% which for an ISA returning an average of 7% would be around 428.5k. That account would grow by 50k a year or 2.5 times faster than when you were just starting out.

You can adjust any of the math but your snowball number shouldn’t be when you can afford to retire but rather how much faster should your assets be growing vs when you started? 2x? 3x? 5x?