In the US we use the slang term "to ride the gravy train" to describe a situation in which someone is benefitting from a situation without having to pay for the benefits.
"He's riding that gravy train" means he is taking advantage of a good situation at little or no cost to himself.
A simple example: a person lives in another's house and enjoys a bed, the food, the TV, the computer, the beer ...all the amenities...for free...at the generosity of the owner.
We would say to him, "Man, you should ride that gravy train as long as possible."
It short, it is a free ride. Is there an equivalent expression in Spanish?