You can hold a hollandaise for about an hour or so, provided you keep it warm. One way to do this is to set the bowl atop the saucepan of warm water you used earlier. Letting it cool can cause it to break, although it can be rescued by whisking in some hot water. Likewise, it can break if it gets too hot (hollandaise is a bit like Goldilocks), in which case whisking in some cold water can sometimes save it.
In no case, though, should you keep it longer than a couple of hours; after that, you should just toss it.