Usually it takes one turn or so of the technology cycle. Basically the market is poisoned as long as it's too competitive for customers to really differentiate between alternatives. Developers realize this, and stop paying attention to the market. Adjacent technologies (eg. iOS platform libraries, hardware capabilities, backend systems, market penetration) continue to improve, but nobody notices their improvement because everybody's given up on the space as unprofitable. Eventually somebody combines 2-3 of these new emerging technologies together in a way that makes a big difference to consumers, their growth rate starts to look like a hockey-stick, and they get bought for a billion dollars or so (if they weren't already in a big company). Then people start paying attention again, you get another gold rush, etc.
We went through this with the web between 2000-2004. All the money-oriented folks gave it up for dead with the dot-com crash, a number of tinkerers started to move in and play with it, and eventually we ended up with the GMail/GoogleMaps/Flickr/Facebook/YouTube/Reddit/AJAX/Rails explosion around 2004-2005. So usually 3-4 years.