most accurate answer would be chosen by me (:
thousands of species
not all make webs
those that do can vary from half hour to one day
depending the specie
some live in caves and only make doors
other only hang on a thread
other make temporary little webs
others make huge webs to last for weeks
some make a web when they all combine (gossamer)
some just walk and hunt (wolf spiders ,tarantulas)
you should make a question much more specific
also it differs from climate.
bird spiders make huge nets between trees
that take maybe several days to make
when the web is a cobweb ,it is not used by the spider any more,they are old abandoned webs full of dust
Cob webs aren't made by spiders. They are dust accumulations.
Spider webs vary by species, so it's impossible to say how long it takes unless you specify which species.