It depends on the size of the tank, the number of otos, and the amount of algae available, but normally they will consume most of the algae relatively quickly. Is this a planted tank?
For feeding, algae wafers should be fed only occasionally. Even the better brands contain a lot of protein, which isn't the best for a staple oto diet. As far as brands go, HBH is highly recommended since it contains spirulina as the first ingredient (as opposed to fish meal). Feeding just algae wafers would keep them alive, but it is not a good staple diet for long-term otocinclus health.
The majority of their diet should consist of various vegetables (when there isn't sufficient algae available). Mine enjoy cucumber, zucchini, and brussel sprouts the most. They also eat romaine lettuce, shelled peas, and baby spinach. They will eat the baby spinach eagerly but for some reason don't care for the larger spinach leaves.
Another thing to try is 'farming" algae for them. You can do this by putting rocks into a clear container with some fish food, and keeping it by a sunny window. When algae forms on the rocks they can be placed into the tank for the otos. You can keep rotating rocks this way to give them a steady supply of algae.
