When it is time to use Hadoop?
It's an economic calculation, really.  When you have a computing "problem" (in the most general possible sense) that you can't solve with one computer, it makes sense to use a cluster of commodity machines when doing so A. allows you to solve the problem, and B. is cheaper than a forklift upgrade to a bigger computer, or upgrading to specialized hardware.

When those things are true, and you are going the "commodity cluster" route, Hadoop makes a lot of sense, especially if the nature of the problem maps (no pun intended) well to MapReduce.  If it doesn't, one shouldn't be scared to consider "older" cluster approaches like a Beowulf cluster using MPI or OpenMP.  

That said, the newer YARN based Hadoop does support a form of MPI, so those worlds are starting to move closer together.