It's time to move on once you have made some sort of improvement in either efficiency or distance and it has become part of the way you now throw a disc. So, for example, if you make a change and now are throwing 20' further on average or you are throwing the same distance with 75% of the effort you previously put in to your throw then it's generally safe to say you have made a positive change. You shouldn't move on until you have made the change a permanent part of your throw. That could be a few days or a few weeks. You just have to get to a place where you can unconsciously repeat your "new" form.
And if you want to know if you have implemented a change correctly and it's something you should continue to build off of then post some video. This is all trial and error and the more changes you make the more easily you will be able to tell if you have made the change permanent and if it is a positive change.