It's hard to say, but it seems like most of the time when you attack, the charge counter keeps going and if you win the attack, however many 100% on the charge counter, you get that influence. So if a battle takes 20 minutes and it's a 1v1, you get 5% for the win + 10% for two charges (1 every ten min).
That's great.
But when you defend or counter attack, because you have no charge counter it seems like it doesn't work the same way.
I just had 2 aeons attacking planet 856, a UEF planet, so I attacked them. 46 minute long game and I won. There two two of them so I should have gotten 10% from that, and their charge counter had to have been above 400% or even 500% if they had some time on it when we started (and accounting for minor lag). So I should have gotten 10% for beating two enemies and +25% for the charge counter, so +35% influence. Instead I get 5.