I've been doing the same thing, more or less, for a few years and progressing in lifts and weight. I consider a plateau or stall when a particular lift of mine isn't moving over several weeks.
At that point I consider if all my lifts are stalled or just a particular lift. At that point some introspection takes place-- am I being lazy about that lift? Is it diet? Attitude?
Then, I make an adjustment. Maybe back off and do higher reps, or more volume in general, or less. Maybe change a grip. Maybe stricter form for the lift. Maybe looser form. Maybe just change my attitiude and attack that lift. Maybe train that bodypart more frequently.
Rarely, if ever, do I tank a whole routine because I think I hit a "plateau".
Changing up every 6-8 weeks? That almost seems ridiculous to me. I would barely even be able to tell if something was working in that timeframe, let alone make the decision that it's "stale".