To give exact dates...
The last GOP primaries are Cali, Montana, New Jersey, New Mexico, and South Dakota, on June 7. The last Democrat one is Washington DC on June 14,
Then the conventions are:
Democrat: July 25-28
GOP: July 18-21
And for the minor parties, which don't hold primaries in most states -
Libertarian: May 26-30
Green: August 4-7
These have been pushed earlier, often the party conventions are in August. From August the general election campaign starts and runs on through August, September, and October, with the actual vote being on November 8.
It's only just hitting me as I write these numbers down just how bat-armadillo insanely long this system takes.