You usually judge when it's ready for harvest by the color of the trichomes. If you have a jewelers loop or anything with macro focus you can look closer. You want the heads to be milky, not clear. If they're clear then you give them more time. The flower is at peak THC when the trichromes are all milky. If you like CBD's you can wait until some trichromes start to amber. A lot of people usually just go with an 80%/20% split on milky/amber.
Online search says flowering time for purple ayahuasca is 55-65 days. At 7 weeks flower is 49 days so you can probably give them another week or two before you cut them down - like I said, it depends on trichromes.