Easy, when it is a new and improved version of the actual Bitcoin protocol - as anyone and everyone would expect it to mean.  We are using the first version of the Bitcoin protocol (1.0) when the next version of the protocol is released that would be called "Bitcoin 2.0"

I wanted to add something here:

It will be Bitcoin 2.0 when it is an improved version of the Bitcoin 1.0 protocol and my Bitcoin 1.0 BTC can be seamlessly and effortlessly converted (or duplicated) into the new version 2.0 BTC in a fair and transparent way (see Peter's proposal).