There’s not really a set amount of time. Some exercises in my chest workout for example are staple, I always start with incline DB presses. Other exercises I’ll rotate sometimes, like maybe trade pec deck for cable flies or something. But, for you starting out, I’d recommend maybe 8-12 weeks to give yourself a chance to get used to the movements and improve strength on those movements, and then rotate. Also keep in mind you don’t need to rotate the entire workout, you can keep some exercises and switch out others.
One thing that cannot be stressed enough is the importance of solid MMC (mind muscle connection) and really using the weight as a tool to maximize muscle tension while still executing with good form. There’s a recent thread on this here: The MMC Thread (Mind Muscle Connection) .