jq 使おう

REGION='ap-northeast-1'
OWNER_ID=$(aws sts get-caller-identity | jq -r '.Account')
TARGET_DATE=$(date -u -d '-2 day' +%s) # 2日前をターゲットに

SNAPSHOT_IDS=$(aws ec2 --region ${REGION} describe-snapshots --owner-ids ${OWNER_ID} \
 | sed 's/\.000Z/Z/' \
 | jq ".Snapshots | sort_by(.StartTime) | .[]" \
 | jq "select (.StartTime | .== null or fromdateiso8601 <= ${TARGET_DATE})" \
 | jq -r '.SnapshotId')

あとは、これを回して消したりなんだり

for SNAPSHOT_ID in ${SNAPSHOT_IDS}; do
    SNAPSHOT=$(aws --region ${REGION} ec2 describe-snapshots --snapshot-id ${SNAPSHOT_ID} | jq '.Snapshots[]')
    DESCRIPTION=$(echo ${SNAPSHOT} | jq -r '.Description' | head -n 1)
    # Description に hogefuga が含まれる場合だけ処理
    if [[ $DESCRIPTION =~ hogefuga ]]; then
        STARTTIME=$(echo ${SNAPSHOT} | jq -r '.StartTime' | head -n 1)
        echo "Start ec2-delete-snapshot : ${SNAPSHOT_ID}[${STARTTIME}]: ${DESCRIPTION}"
        aws --region ${REGION} ec2 delete-snapshot --snapshot-id ${SNAPSHOT_ID}
        if [ $? != 0 ] ; then
            echo "Error ec2-delete-snapshot: ${SNAPSHOT_ID}"
            exit 1
        else
            echo "Success ec2-delete-snapshot : ${SNAPSHOT_ID}"
        fi
    fi
done