i think that Twilight was only a few months, like 6 months, not very sure
but New Moon started with Bella's birthday, 13th of September, so i assume that about a year has passed between Twilight and New Moon. Then New Moon goes for a little less than a year, since Edward apparently returns after 8 months
and then eclipse is actually only a few months long, since half of the events occur in a few days' span (they keep mentioning there are a couple of weeks to graduation)
So all in all, the whole events of Twilight, New Moon and Eclipse occurred in the span of about 2 years (at least she finished 2 grades in school)
Breaking Dawn is about 5-6 months, ** SPOILER ALERT ** because the wedding is on the 18th of August, one month before her birthday, and then she is turned into a vampire 3 days before her 19th birthday which is the 13th of September, so basically between New Moon and Breaking Dawn (half way through breaking dawn) is a year
then there is the volturi issues, which happen like 4 months later (3 months pass, then alice sees the vision of them coming a month later)
then the confrontation is the last main event happening
so the span of all the FOUR books, is like 2 and a half years, or something like that
which is actually quite ridiculous