Hi,

This is simply calculated by :

Delta T' = Delta T / Sqrt (1 - v**2 /c**2)

where v is the velocity of the traveller, c the speed of light,

Delta T' the time passed on earth and Delta T the time experienced by the traveler.

Taking Delta T = 1 year and v = 0.99 c , one obtains Delta T' = 7.1 year. If v=0.999 c, one obtains Delta T' = 22.4 year.

So, in principle the time span on earth can be multiplied by an arbitrary large factor when coming closer and closer to the speed of light (in the limit, infinite).