there are tons of factors you can manipulate to give your training a kickstart as an example you can change your workouts from working muscles to something like push/pull/legs or upper/lower or total body. also you can change the exercises (not the staples really, but there are tons of variations to the staples that you can change), the sets, reps, tempo, rest intervals (AKA density), you can deload, you can do more advanced stuff like increasing volume over a couple of weeks then deloading. Do some reading on this site, read the stickies and do searches on periodization. Now i don't know where you are in terms of what you know about exercise science and all that comes into play with it, but if you keep reading and educating yourself it'll all click.
"The greatest obstacle to knowledge is not ignorance but the illusion of knowledge." -Barry Marshall, Nobel Laureate
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Right now, every 8-10 weeks (provided I enjoy the program), but I'll usually take some sort of break in the middle whether it's a different routine, lowering volume/intensity (active recovery), or just doing nothing for a week.