I was watching a documentary that speculated that a space sail could propel a space ship at "millions of miles an hour". The documentary hypothesized that this technology could allow humans to colonize habital planets around distant stars by reducing travel time to "generations". To wit I say better move a little faster. Feeling arbitrary, I picked 5 million miles an hour. Here's the distance in miles to the nearest star Alpha Centauri which is galactically speaking "right next door" a mere four light years away -- 23462784000000 (23.4 trillion). It takes somewhere around 535 years to get to Alpha Centauri at 5,000,000 mph. That's about 20 generations, or about an eighth of recorded human history.

186000 << miles/second speed of light
31536000 << seconds in a year
5865696000000 << Miles in a light year
23462784000000 << miles to alpha centauri (4C) 23.4 Trillion Miles
4692556.8 << hours to alpha centauri at 5,000,000 mph
195523.2 << days to alpha centauri
535.68 << Years to alpha centauri