Historically, we learn of the GOP VP nominee the week before Convention, where the whole ticket is formally announced. Barack Obama announced Joe Biden as his VP 7 days prior to the Democratic National Convention. John McCain announced Sarah Palin as his VP nominee 3 days before the Republican National Convention.
George W Bush announced Cheney as his running mate 4 days prior to the convention; John Kerry announced John Edwards as his running mate 20 days prior to the convention (which was considered much too long). Bill Clinton announced Al Gore as his running mate 4 days prior to the convention. Al Gore announced Joe Lieberman as his running mate a week before the convention. In 1992, George HW Bush announced Dan Quayle as his running mate ON the first day of the convention (which was considered too short notice).