How long before robin no longer needs the nest on my porch?
According to Cornell's All About Birds website, you will have to wait about a month for the nest to be cleared.


The egg incubation period is 12–14 days.
Following hatching, the nestlings will remain in the nest for another 13 days (i.e., the "nestling period" is 13 days).


However, there are two caveats to this:


A typical robin clutch size (i.e. the # of eggs in the nest at once) is 3-5 eggs. Since you only have two eggs, that could suggest that more eggs are about to be laid very soon, which may extend the nest occupancy another day or so.  
Some robins use the nest more than once!


  An American Robin can produce three successful broods in one year. 



Here's a visual of what the nestling period (approximately) looks like so that you can track your nest visually :) :



 Source: L. Birnbaum