Every nation gains influence every update (12 AM and PM EST), so long as the user logged in within the last seven days. In the case that the user doesn't, influence will stay constant. Non-WA nations and nations with no endorsements will gain 1 SPDR point every update. While nations with endorsements will gain 1 + their number of endorsements every update. Therefore, a nation with 400 endorsements will gain 802 SPDR within a day, whereas a nation with 0 endorsements will gain 2 SPDR a day.
What is regional influence?
Regional influence is the automatically-generated measure of influence a nation has compared to other nations in their region. It was created in 2006 in response to the ability of players to eject extremely large numbers of nations out of a region, and password it. Therefore, influence only really matters in a gameplay sense, rather than an in-character/roleplay sense, unless you specifically make it so it does (i.e. by electing the most influential player of the region to a regional position).
The current regional influence levels are as follows: