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The rule of 72 is finance's most famous mental shortcut: 72 divided by the annual return gives the years to double your capital. It also works in reverse, showing how fast inflation halves purchasing power.
Indicative calculation for educational purposes: not a forecast nor financial advice.
At 6% capital doubles in 12 years, at 9% in 8, at 12% in 6. The rule approximates logarithm math and is surprisingly accurate between 4% and 15%.
The dark side: the rule applies to inflation too. At 3% a year, idle cash loses half its purchasing power in 24 years; at 6%, in just 12.
Very, for returns between 4% and 15%: the error versus the exact calculation stays within a few months.
It's close to the mathematically correct value (100·ln2 ≈ 69.3) and divides neatly by 2, 3, 4, 6, 8, 9 and 12: perfect for mental math.