Many people find a good workout routine that they like, that’s giving them some good results and they get accustomed to. Many times you end up doing basically the same workout it over and over since you were getting some good results. The problem with this, like everything else, is your body gets used to it. Eventually your gains plateau, and your workout that was giving you all those gains eventually is giving you no progress at all. While many people wonder why they aren’t getting gains anymore when they are working very hard, it may be because they are doing the same thing over and over again.
It’s good to switch your workout routine when your progress plateaus out, your workout are starting to feel monotonous and you are starting to lack motivation. Usually, this is about every 6-8 weeks. I don’t recommend changing it much less often, because you need to let your body get some gains from the current routine you are on. Some people like to switch things up weekly or even daily, which is fine, but if you are trying to get stronger for example on a certain exercise, it may be difficult if you aren’t doing that same exercise for a few weeks in a row.
While there is no exact timetable of when to switch out your workout routine, usually your body, your mind and your lack of results will be the key indicators. If you aren’t getting the feedback of when it’s time to change, figure every 6-8 weeks, maximum 10 weeks to start a new workout routine.