Each campaign receives an eCPMi as a combination of bid and campaign CTR.
This eCPMi is recalculated hourly for 7 days rolling (168 hours). It's updated if a campaign had at least 10 clicks or 100.000 impressions over this period (rolling week). All available campaigns are then ranked by their eCPMi value.
An individual eCPMi is calculated for each publisher ID (provided that a campaign has received enough data from a publisher).
Please note again, that eCPMi is only calculated on campaign level (not for a specific advert).