Are review credits cumulative?

If I post multiple reviews in a short amount of time, am I awarded 6 weeks of premium membership for each, or only 6 weeks from the last one?

In my experience, 6 for each.

yes they just add on

Click on "User CP" then click "Paid Subscriptions" on the left of Eccie Homepage. It will show you the end date of your Premium Service. It is cumulative.