There is no universally correct answer for everyone in this situation; it depends on factors in your relationship, your comfort level, and the possibility of changes in your sex life. However, if your partner is taking HIV medications and maintains an undetectable viral load, and he is your only sexual partner, there is no risk of him sexually transmitting the virus to you. This concept is called U=U (Undetectable = Untransmittable). If you do sometimes have other sexual partners, or may in the future, continuing PrEP will continue protecting you. It is also perfectly reasonable for you to continue PrEP as long as you would like if it helps you feel protected, or if you are unsure how comfortable you are stopping.