It is a research chemical, so its long term effects are relatively unknown. I can tell you from my own experience, that you start getting diminished returns over time. I would limit it to 8 week runs maximum, and most of the time 4-6 weeks should be plenty for your rats.
Yes, to be safe, I would never run more than 8 weeks. That is kind of a general rule of thumb with most sarms and I would treat GW the same way until there is a better understanding of it through study.
If you don't have a bottle, I highly recommend Sarmssearch. Nothing else compares in GW quality.