Well, its hard to expect scientific accuracy from Discworld series, but can we at least speculate? We know that between "The fifth Elephant" and "Rising Steam" passed about 8-9 years (Sybil just found that she is pregnant in the former and young Sam is about 8 in the latter). We know that some time had to pass between "The Colour of Magic" and "Mort" because Ysabel has grown up, but she was living in the land of Death where "time is irrelevant", but we know that at least 16 years has passed between "Mort" and "Soul Music" (Susan is 16 and she spent most of her life in normal time) and then in her 20s in "Hogfather" and "Thief of Time".
So skipping the "Pyramids" and "Small Gods", that both happen completely out of other series, how many years has passed between the earliest (in universe)and latest Discworld book?