Hi Jude,
Welcome to our community!
No, there is no time limits for using Japanese Knotweed. I took it for an entire year back in 2011 and it helped to heal me from a 40 yr old Lyme infection.
But don't take my word for it, you can see it here on Buhner's site (a Lyme literate Master Herbalist):
buhnerhealinglyme.com/category/herbs/knotweed/ There are several questions about
the safety of Japanese knotweed in that list.