I have never flown a trainer so I have no idea how it flies or when you know when to move on. I do however think that, IF you are honest with yourself, you will know when you are pretty good with a trainer. I think every person judge by different levels and things. For one person being capable of handling a trainer at a lower level - will feel they have mastered it - because perhaps they are only interested in doing lazy circuit flying and just enjoy doing low and slow fly bys. Some peole will have an higher level at which they judge whether the plane has been mastered - because they want to go extreme with planes (3D and fast hard flying for example).

In other words, I think, it's what type of pilot you are which will determine if you have mastered it. I think it has more to do with - when have you mastered a certain type of flying. This then also leads to what the planes capabilities are. There are too many variables that influence an accurate answer I think.