How long is 1 guinea pig year?

A guinea pig's life expectancy is in the 3..5 year range (well cared-for, in captivity), that of a human is approximately 80 years (in the developed world).

Therefore, each year of a guinea pig's life equates approximately 80 / 5 = 16 years.

Note that simple linear adaptations do not provide a good translation. For example, a guinea gig reaches fertility within weeks, not within the human life equivalent of 14..16 years. The true translation between a guinea pig's age and that of a human will be non-linear, where the guinea pig gains more "human equivalent years" rapidly at first, but then ages at a slower rate than initially.