On which week should I start using PK fertilizers ...

It is best to start boosting p and k as soon as she transitions into flower. The trick is to slowly back off of the grow fertilizer as you add more flower nutrients over the first few weeks of flower. The leaves are looking pretty dark green now with some burned tips and leaves clawing down which says she is getting too much fertilizer. Particularly nitrogen. I would give her a good flush on the next watering with plain water then start her on some bloom nutrients to help build the buds. The grinspoon is a challenge even for the most seasoned growers because it is a long flowering time to keep her healthy but you can do it. Just takes vigilance in monitoring her leaves and giving her what she needs. This is a marathon rather than a sprint. Take is easy on the nutrients because it is easy to add a little more feed but impossible to repair damaged leaves. Good luck!