Proxima Centauri is about 25 trillion miles away, and the fastest man made space object to date (one of the sun probes from the 1970's) reached speeds just greater than 150,000 mph. So let's say all of mankind teamed up and invented a spaceship that could go 1 million mph then you looking at 25 million hours or about 2854 years. For a manned trip even in a generational ship (you blast off from earth, but your great, great, great, etc. grand kids reach the destination) it would require much greater speeds. At half the speed of light, roughly 335 million mph it would take 75000 hours or 8.5 years. Current estimates for mankind attaining anything near those speeds are around 5000 years from now. But it could happen much sooner, in 100's of years, it's within the realm of possibility. Just imagine explaining to someone around the time of the American revolution about a 7 hour flight from NY to London or Computers.