Everyone will have an opinion on this here's mine...
You have master your trainer when......you grease in 95% of your landings
                                                                      You can fly the entire pattern inverted
                                                                      you can do more than one roll without loosing altitude
                                                                      and lastly you feel that you have gone just about as far as you can go with your trainer...
Whatever you do hold on to your trainer for those days when you just wanna be lazy and shot a tank full or two of touch and go's