select sum(total_amount) >= 12000 over_12000
from mytable
where customer_id = nnn
and date_added between 'YYYY-MM-DD' and date_add('YYYY-MM-DD', interval 6 month)
Replace nnn with the particular customer ID, and YYYY-MM-DD with the start of the 6 month period that you're interested in.
UPDATE:
This will use the 6 month period leading up to the customer's last order:
select sum(total_amount) >= 12000 over_1200
from mytable join (select max(date_added) last_date
from mytable
where customer_id = nnn) last
where customer_id = nnn
and date_added between last_date and date_sub(last_date, interval 6 month)
UPDATE 2:
select m1.date_added end_date, sum(m2.total_amount) >= 12000
from mytable m1 join mytable m2 on m2.date_added between date_sub(m1.date_added, interval 6 month) and m1.date_added and m1.customer_id = m2.customer_id
where m1.customer_id = nnn
group by end_date
order by end_date