Time Dilation

Time Dilation, Lorentz contraction, Lorentz factor, velocity, speed of light - all that fun stuff

If you travel at
70.71067811865475 % the speed of light

or

0.7071067811865475 as a factor of C

you, if on the ship, would travel 1 light year in, what appears for you, to be 1 year.

In short: this is the magic number for time dilation, lorentz whatever, and so forth. In short; that's a good speed to travel at. 

The dilation you'd experience is 41.42135623730948% meaning that a 10 year journey (from your perspective, on the ship) would take (from the perspective of people on earth) 14.142135623730948 years. That's roughly 4 years and 52 days longer than 10 years. 

While that is a long time, it is not something that would destroy a person. A "career" on a space ship might see you travelling at these speeds for 21 years, which would only be 30 years. This means your twin would be 59 when you are 50, and while that's unusual, it is not as bad as it could be if your twin was 59 when you are 21.