Classical computing has a 50 year head start, but development in quantum computing will likely be faster (we already know how to miniaturize, and we have computer aided design and manufacturing tools). But not so fast that we won't see problems coming decades in advance and have plenty of chances to switch algorithms.
+1I think I've said it before, but I'll say it again: I'll start to worry when there is a quantum computer that can factor 64-bit numbers faster than non-quantum computers. I bet that is at least 20 years away...