Why the answer varies
Training experience, exercise selection, effort, frequency, sleep, nutrition, other activity and individual response all change how much work can be productive.
A decision process
Keep exercise selection and effort reasonably stable, establish a repeatable weekly baseline, and observe several weeks. Add work only when performance and recovery indicate that the current amount is being handled and the goal needs more stimulus.
Signs to inspect before adding
Repeated performance decline, joint irritation, missed sessions, persistently worsening effort at the same work or a programme that no longer fits the week all deserve inspection before volume rises.