You use hence in expressions such as 'several years hence' or 'six months hence' to refer to a time in the future, especially a long time in the future.
1. (used to introduce a logical conclusion) from that fact or reason or as a result;
"therefore X must be true""the eggs were fresh and hence satisfactory""we were young and thence optimistic""it is late and thus we must go""the witness is biased and so cannot be trusted"