If you are playing the regular puzzle mode, there is no need to die. The only thing that would be guaranteed to do you in is that eventually you will use up so many words that there won't be enough left. But even that is not a problem, because I have discovered that the memory cache for the words already used is too small, so that if you play long enough, you can start using the same words a second or third time. This starts kicking in after you have amassed about 600,000 points.
Surely someone thought of that scenario when developing and tested it... Right? No?