"51 Days" joke

Three blondes walk into a bar and ask the bartender for three shots of tequila.
He looks at them and says "OK" and pours their shots.
They all clink glasses and and yell, "51 days!" Then they proceed to slam the
shots, looking very self-satisfied. They look back to the bartender and decide
to order another round. This time they pick up the glasses again and, more
gleefully yell, "Only 51 days!"
The bartender finally can't stand wondering what they are talking about and asks
them what they mean by only 51 days.
One of the blondes looks at him and says, "Well," looking very smug, "We just
finished a jigsaw puzzle. It took us only 51 days... and on the box it said 4-7
years."