In terms of moving on from one technology to another I have found that the best indicator is when the large consulting firms start hiring lots of staff with a specific technical skill, most of them poorly qualified for that technology. If you prefer to contract directly to the larger client firms, then the big firms start being your competitors.

In my experience high demand and high rates environment lasts 3-5 years. Bit of a challenge to pick the next best technology. Not too soon on the adoption curve and not too late.