Could take you a while
I reread the entire series (around forty books I think) back-to-back a couple of years ago and it took a few months.
The first few (starting with "The Colour of Magic") are fairly light (Terry Pratchett himself said that it took him a few books to get the idea of "plot", as far as I recall) and mostly just intended to be humorous parodies of the fantasy genre, but later ones are often also a satirical commentary on what he perceived as the injustices of modern society. Reading the first two or three does help to provide a background for the world he was writing about, but after that it probably doesn't make much difference if they're read out-of-order. I've always enjoyed them having read the first in 1984. I think some people can be a bit sniffy about them because they're often labelled as "comic fantasy", which misses the point entirely.
Within the series there are groups of books based around common sets of characters, such as the witches, or the wizards, or the night guard, as well as ones more aimed at a particular theme, but the characters aren't restricted to the groups specifically relating to them.
If you do enjoy them then "Good Omens" (co-written with Neil Gaiman) might also be worth a read. It was made into a television series (by Amazon?) a few years ago starring Martin Sheen and David Tennant and is worth watching just for the chemistry between the two of them.
James