I am not sure if anyone can tell you how long it takes to reach various non-commissioned or warrant ranks. It depends entirely on factors such as, obviously, ability, conduct, variation in sizes of a regiment, casualties. There is no pattern that sets out that a certain rank is reached after so many years of service, and never was. If he reached the rank of C.S.M. that was awarded by Royal Warrant so he was clearly a competent soldier - and literally 1 in a 100.
By the way, it is the Boer War.