Hi Zara,
Our gp told dh 7 days which got a prompt 'you must be joking'!
It ended up being 3 days, which I think is correct and got 40mil per ml. He wanted to better it so the next sample was 5 days worth and got 150mil per ml. (even the doctor was impressed).
You could always phone your local hospital's pathology lab.
Love cath xx