In my experience there needs to be alot of evidence. I have reported (mandatory in my work) a number of times over the years, and have been told many times that they gather reports from a variety of sources. The more reports from a variety of sources, the faster the case is acted on. That's why, even if you feel something you have observed isn't THAT bad, when put into consideration with something several others may have reported, a clearer picture is built - and it's very important to actually tell someone.

That said though, if there is only one piece of evidence, but it's a big piece (like obvious bruising or something) the wheels will move much faster - usually within hours.