Honestly it can take years for a company to make a transition to being agile. Teams can often make a transition to Scrum within a matter of weeks.
There are practices and processes and it takes time for all the pieces to come together. For instance, Agile software development teams often don't have the capacity or capability to turn on CI or adopt TDD overnight.
Staying on the right track is the hard part - for instance, if teams start getting over-confident and missing their retrospectives they may quickly find themselves back at step one. In fact, skipping retrospectives and not looking for opportunities to continuously improve the team is one of the biggest reasons for a failed Agile adoption in my experience.