Does dopamine signal become stronger when goal distance is defined using time?
Hamid et al. Mesolimbic dopamine signals the value of work. Nature Neuroscience 2016.
http://www.nature.com/neuro/journal/v19/n1/full/nn.4173.html

Here is a paper that argues that dopamine signals discounted value of future returns. This is precisely what you are asking. 

the authors argue, that if rewards are further away in time or space, they are discounted, and that DA release in the ventral striatum is more strongly related to this discounted value than prediction errors (current dominant theory of DA function).

I think you should take a moment to read it! It is a seminal paper that makes a profound point about dopamine function, very neatly unifying previously disparate ideas of what the hell is DA good for.