I have three otocinclus in my 10 gallon planted tank, and they did a great job on diatoms at the beginning, and now keep my tank pretty well algae free. I feed them a slice of cucumber once a week (leave it in for a day or so), and that seems to keep them healthy.
I recently started a new tank (28 gallon), and had diatoms show up about 4 weeks in, as well. I have just this week started to see the diatoms disappear. The tank has now been running for a couple of months, so I would say the diatoms lasted about 3-4 weeks. I just kept wiping them off the front and side glass, and would rub what I could off the plant leaves.
So, you can either add some diatom eating fish or snails, or wait them out.