Configure frozenTimePeriodInSecs in indexes.conf. This is the point at which data is tossed from the index. You will also want to configure maxTotalDataSizeMB to keep your disk from filling. Just in case.
Additionally, you should make sure the amount of data in each bucket is appropriate. If you have the max time period defined to 3652460*60 = 31536000 seconds, it will only delete when the bucket rolls over to warm, and the newest event is older than 1 year.