The weight of the dumbbells lifted in the manga isn't fixed. It depends on the plot and the characters' training goals. Sometimes it's just a few kilograms for starting out, and other times it could be dozens of kilograms for advanced stages.
The dumbbell weights in the manga are all over the place. It depends on who's lifting and where they are in their fitness journey. Beginners might start with light ones, but as they get stronger, the weights increase significantly.