When should one learn a new technology?
The short answer is Now!
Not now as in ooh-shiny! but as in I-have-to-learn-this-anyday-anyway.
Take this example: Linq.
Linq is a sterling technology for writing code that is easy to read and have fewer bugs. It takes an hour or two to grasp but is payed back when the first bug is avoided. Here is the cherry on top: you will have to learn Linq one day. So why keeping on building your technical debt?
Is it smart? No.