Davy,
It will of course depend on the initial dose but if you keep it low, once a week should do. What is best is to grow high density culture of algae first, than let it sediment and pour out enough of the cultivation sulution to be able to dilute it in like 1:5 ratio with clear water. Growing the culture should not take long when you start with 10% of inoculum. Than put the aeration back in and wait for about 3 days. This way you are able to starve the algae from macro elements and it will make them prepare for “bad times”. What they do than, is they start to produce reserve substances like HUFAs and PUFAs and other lipids or other valuable nutrients that are not present in the algae cells to nearly such content when the algae have enough nutrition. Of course by limiting the macro elements, you cause the culture to stop the reproduction or it will reproduce much slower but you will get much more valuable algae and therefore much valuable Moina.