Just a few days
In fact, if Tang Yin used the more literal meaning of 'Source of Power', he could think of countless different subjects.
After all, what does a source of power do in its most literal meaning? It provides unlimited energy and power. As long as it is there and nothing affects its existence, it can generate and deliver energy and power.
If he were to use that as an example and raise it to the level of what a Primordial Law is, the Law of Control could generate an unlimited amount of power for him. Either for his own cultivation or someone else's cultivation.
Furthermore, also for the creation and strength of a skill or power cast. The thing is, that's where the issue of tools comes into question.
If it is thought that some tools cannot draw 100% of the user's power because they would break, then something similar should exist in this case.