I'd say every 2months roughly...
A guy I spoke to recently explained to me that people who after 3-4 weeks stop seeing results at the gym freak and think they gotta switch routines... so they run around trying to find new exercises to hit the muscle differently... his theory was its not really about that, its all in the end about increasing that 1RM... so switch up the set/rep amounts for example, so your body isnt used to it
i just finished 8 weeks of 5x5s, am now going for 3x8s (had to lower weights a tiny bit bit I kinda feel more of a "pump"when i do my exercises)... in another 2 months im not sure.... maybe go back and do a stricter 5x5 or possible go for 12rep range on lighter weights but mega mega uber slow and see what i get out of that (probably more of a increase in muscle mass rather than strength) but if so then all is good, ill just go back to low rep power training after, i think mixing it around is a good idea...
correct me if im wrong!