Of course there are diminishing returns but that can only be decided by the trainee themselves. Having done 600 swings with the 24 kg I know I don't want to do more if it means training longer, so I will now try to overload by doing the same work in less time.Over the course of a training cycle of course. I know I am adapting because it wasnt long ago that half that workload would make my legs sore the next day. Now I can do double what I used to and have no soreness the next day. Adaptation. One critical key to progess.
That last part is no mean feat and everybit as important as anything else you will train.Because when it all comes down to it, it's that little battle between the weak you and the you that wants to be stronger that is the first overload. Choose wrong and you will get weaker and less; choose correctly and you at least have a chance of being better.
Assuming you have some.