how long a light year on earth will seem
You can never reach the speed of light but can get very close to it. Thus, when you travel with, say, 99.99% speed of light it will take you a little more than 31 years to reach the star 31 light years away. This is, however, the time it takes seen from earth. In the spaceship, the time taken will be dilated by
$$\gamma ={1\over \sqrt {1-{v^2\over c^2}}}$$
Which is roughly a factor of 70.7. This means in the space ship it is only 0.44 years or 160 days.