Compare the annuity and perpetuity formulas. The difference between them is the 1 − 1/(1 + r)t term.

To be three-quarters of the value, this term has to be 3/4. So you must solve 1 − 1/(1 + r)t = 3/4, or 1/(1 + r)t = 1 − 3/4 = 1/4 or (1 + r)t = 4. Taking logs, t = log(4)/ log(1 + r). In the main question, r was 5%, so t = log(4)/ log(1.05) ≈ 28.41 years. More generally, to reach a given fraction f of value, t = log[1/(1 − f )]/ log(1 + r). Think of this number of years as helping you judge the quality of the infinite-period approximation in the real world. If it is more realistic that you have fewer than 30 years of cash flows instead of an infinite stream, then the perpetuity formula may not be a great approximation of value when the interest rate is 5%.