Yes, it's hard to even see them.... Infusorians? You can't tell. Micro worms? you might see their tummies get a little bigger with a light color.
How many days? Really the first week their needs are few as they still have some of the yolk sac left that they are feeding on, but they also start "testing the waters" for outside food.
Have you tried a few bbs? Perhaps they can eat it, perhaps not, but you can tell when they do eat them as their bellies turn pink.
Youtube has plenty of vids of babies eating mw and bbs...