Just trying to do some math gymnastics to understand how the previous solutions worked (e.g. puzzle 53, 54, 55) using only bitcrack (not kanagaroo or other methods) and a fast gpu e.g. RTX 2080
(assuming 1 Billion keys generated per second)
Is my maths correct:  

15 characters in 100,000 seconds (28 hours)
16 characters in 1,000,000 seconds (278 hours) 11 days
17 characters in 10,000,000 seconds (2777 hours) 115 days

Calculated by converting the hash to a long number (e.g. 15 characters = 15 digit long number) and dividing it by 1 Billion keys per second to see how long it would take to sequentially reach the final value.

However if the above is correct, then how come puzzle 64 hasn't been solved within 115 days? or faster assuming some users are using multi-gpu? i'm sure my calculation has gone wrong somewhere above but not sure how