My own experience - across multiple teams, multiple organisations, multiple languages and frameworks - is that it can take weeks to evaluate even relatively slight changes to existing methodologies.

Let's say the OP decides to adopt Extreme Programming.

How long would you say that a team with zero experience in that methodology would take to get to the point where they're practising it well enough to perform a useful evaluation of the consequences?

Less time than it would take for you to realize that our experiences form only our perceptions, not universal law.