6-8 months ... hmmm! I myself would not follow a guidleline like this. If during this time you are still seeing results then why change to something else? Results is what we all strive for so when you have something that works why would you change it?
When you stop seeing results then this is when you should change things up. You can change things by doing minor things such as less/more rest time, less/more sets, less/more exercises, changing the order of your workout, change out those exercises that are not working and also of course, change the workout itself. Sometimes a new setting can help and by this I mean if you work out at home and then go to a gym and do the same workout, you may see results. Listening to certain music has helped me in the past. What I ate prior to working out has also affected me, not to mention what I ate after the workout. Changing how my brain works has also affected me and by this I mean I was in a slump and the only reason I continued working out is that I did not want to quit what I've worked so hard to do. I was going through the motions only and was not into it as much as I was before. By changing this around and getting refocused and taking some time off (i.e. 1-2 weeks), I was able to do the same workout and see results. This brings me to another change ... rest; taking time off.
It can be a little or a big change but only change when you are no longer seeing results. If in that 6-8 weeks you are seeing results, stick with it and count yourself lucky. Hold onto those results and revel in them. As the years go by and the older you get, results can be slim and slow and anything that makes you better you count yourself lucky.
"IF I COME ... I'M BRINGING THE PAIN WITH ME"