We had Salesforce create a Health Assessment report for us.  Found that we had 6 Apex Classes / 2 Apex Triggers / 5 Visualforce Pages that were using old versions of API's.  Within the "New Code using old API" section we had 37 Apex Classes / 2 Apex Triggers / 4 Visualforce Pages using old versions of API's.

We are new to this process so any guidance / answers / suggestions would be much appreciated for the following questions:
What are others doing to keep API's as current as possible (would it be a good practice to set time aside once a year to make sure all Classes / Triggers / Pages are using the most current version of API's)?  We are requiring when code is updated to upgrade to latest API, but sometimes code doesn't change often.
Can someone give a guess or rough estimate the amount of time it would take 2 people to go through process of upgrading to the latest versions of API's?  (ie so many hours per change or estimate of 40/80/120 ... hours)
What is the recommended process for Apex Classes to upgrade to the latest API version.
What is the recommended process for Apex Triggers to upgrade to the latest API version.
What is the recommended process for Visualforce Pages to upgrade to the latest API version.
Thanks for any guidance you can provide.  Trying to keep our environment as stable / healthy as possible.

Dan