Time to harvest?

I changed the lights on my 7 girls to 12/12 on April 19th. It's now 8 weeks since the lights changed. I don't know what type they are and the microscope I ordered to check when to harvest hasn't arrived

Am I 8 weeks into flowering or only 7 (since they showed sex)? Am I ok to harvest?

8 weeks is a good rule of thumb to start preparing for harvest.
Those microscopes are $9.99 each at Radio Shack and a MUST for any grower. I smoked some of mine that were harvested with CLEAR triches and it is no comparison to the same plant that I allowed the triches to get Cloudy or Beer colored. AND 4 weeks of CURING really makes a potency difference.
Are the stamens (hairs) turning red or reddish brown yet? That is the first sign you are getting close. I wouldn't havest any buds with white stamens.
You didn't say if you are SOIL or HYDRO and HYDRO grows and matures faster.
You should (and growers argue this) but you should give them 48 hrs total darkness before the harvest day.
You should FLUSH them good with clean clear water with NO nutes for 4 to 7 days IF in soil, and 4 days if in HYDRO grow.
Good Luck, got any pics?
Go to GROWFAQ andr ead up on HARVEST and Trichomes.
peace, bro

Unfortunately I'm in Spain and we don't have Radio Shack and I haven't found anywhere which sells them so I have ordered from ebay.

Is it just the potency which changes or the high?

The stamens are turning brown and have been for a week or so. Are you saying you wouldn't harvest any bud which has a white hair (i.e all should turn brown)?

I have them in soil and have not given any nutes for about 2 weeks.

Pics below (quickly taken just now so sorry for the poor quality). Thanks

I've seen growers here harvest an entire plant at once, but I do it in steps.
I harvest the main cola first and the larger buds taht are mature, and I lower the lights to get the fluff buds to grow and get more potent, waiting for the white hairs to turn red or brown.
I do and did harvest some white haired ones, they are good but not no where as good as the mature buds.