No set time. It all depends on the degree of dysfunction and the skill of the CBT therapist.

A formal CBT program is time-limited, usually weekly sessions for about three months, with homework assigned in every session to help you practice and absorb the new skills you are learning.

Check this link out.
https://www.randyweisstherapy.com/how-long-does-cognitive-behavioral-therapy-take/

Also, check these videos out.
https://www.bing.com/videos/search?q=how+long+does+it+take+for+cognitive+behavior+therapy+to+work&qpvt=how+long+does+it+take+for+cognitive+behavior+therapy+to+work&FORM=VDRE