Dynamics CRM On Demand Updates

Just want to run this by the current MS Dynamics CRM on demand users.  Our infrastructure team is concerned about the periodic updates Microsoft pushes. 4 questions surrounding this topic:

1. How often does Microsoft release minor updates?

2. How often does Microsoft release major updates?

3. Is it mandatory for customers to apply the updates or can customers apply them on their own pace (like when infrastructure such as MS office, SharePoint, OS, is ready to accommodate major updates)?

4. Where can I find online references related to updates?

Thanks for your help in advance.

Tang