Pandemic's excess card rule states that if you have more than 7 cards in your hand at any time, you must discard the excess (according to your preference, of course).
Let's say you're the Scientist, have 5 city cards and 2 special cards (like the airlift, or research center build card). The Researcher comes to town and gives you the remaining 1 color city card that will allow you to cure a disease as soon as you get to a research facility.
Let's also say there's only two players (Researcher and Scientist). And that this is the last color that needs to be cured.
This takes you to 8 cards - over the amount allowed.
When do I, as the Scientist, need to play / discard my excess cards (in this case, let's take the two special cards as the ones to play) so I am not in violation of the excess card rule, but also so I maximize the value of the Researcher's endowment of city cards?
edited to reflect the limit of 7 vs 8, as noted by Gendolkari