We're running our own OpenX instance with several advertisers. Each month, the advertiser's campaign should reset its available impressions, based on whatever plan the advertiser paid. I think this is a pretty common scenario. Question is: how do you manage this? Do you expire the old campaign and create a new one with the new amount of impressions? Or do you just add the impressions to the existing campaign?
Managing OpenX campaigns for several advertisers
advertising;openx
I believe in 2.8 the only way to reset metrics would be to clone the campaign and relink. You could also leave max impressions blank and just monitor to make sure it doesn't go over, and then pull your report to show impressions per month or whatever interval you like.