When can I expect an effect?

This varies greatly. In most ponds, you will see a noticeable improvement after 3 to 4 days, while in others, it may take 3 to 4 weeks.
If your pond contains a lot of nutrients, has been chemically treated before, uses a UVC lamp, or still has residues of algae control agents in the water, it may take up to 6 weeks for everything to return to biological balance.
A second application after 15 days would accelerate progress here.