First time flowering with 100%-known strain. Package says 7 weeks of flowering. Is that 7 weeks from the switch to 12/12 or 7 weeks from first sign of buds/flowers?
You should start counting from switching to 12/12 and even then it's only a guide. Actual peak potency and ripeness will depend entirely on your skill as a grower, your environment, your lights and your nutrient feeding. When you actually 'chop' your plant will depend on what kind of 'high' you're looking for. Chop time is a balance between peak ripeness/yield and potency. Eg if you chop early (when the spheres are mostly clear or just turning cloudy) then you'll get more of a 'head high' but will also lose yield, because you're denying the plant the chance to put on more weight. If you chop late (ie when 20% of spheres are amber) then you'll get more harvest but also more of a couchlock to your stone. This applies regardless of whether your plant is Indica, Sativa or hybrids of the two.
Pistil colouring (ie turning from white to red/amber/orange) is a sign of 'ripeness' or how much longer the plant will continue to produce buds. You want to be shooting for about 3/4 of pistils turning colour for an indication of peak ripeness - but this may not correspond with the trichome sphere colour you want because as above - it's a balance.
A good indicator is the '40/60 Theory' this is a useful technique for when you're growing plants of 'unknown' origin ie you don't know what the strain is or what the flowering time might be. The 40/60 theory involves measuring the plants at the start of flowering and as soon as they start stretching (ie getting taller) note the day it starts. Then keep measuring each day until the stretch slows up or stops completely and note the day this happens. At this point, 40% of harvest time has elapsed and you have about another 60% to go.
So If the plant starts to stretch on day 8 of flowering and then stops or slows at day 26, then the plant 'should be' at or around peak harvest on day 65. So if 26 days = 40% then 10% would be 26/4 = 6.5 x 10 = 65 days.
There's a couple of very good threads around on 'potency' and harvest time, you might want to seek them out.