To your questions — broadly — one needs experience, knowledge, gravitas, tools, communication skills, connections and exposure to become a CMO. Let me cover each one and I'll include your questions:
You need a requisite amount of experience to become a CMO. It's not 'how many years', it's more about how many areas do you feel comfortable in the marketing arena? I come from a marketing background — you need creative, technical, statistical, client, financial and communication experience to make CMO. You need to know all the areas you will manage — so many newly minted CMO's who hit the top rung usually are lacking in many of these areas and it shows in their performance. I'm not saying you need to be an expert in all areas — although you should have leveraged each one during your career.
It's what you know. Where has the industry been and where is it going? What is your competition doing? Where has your company failed and succeeded? What CAN you do to move forward? Knowledge is power and when you're at the pinnacle of your organization, you need to have a adequate grounding in the past, present, and future of your industry. People will be looking up to you and it's imperative that you have a good base to lead them forward. Or, surround yourself with people who do.
Leadership happens instantly. You can't train for it — I can tell a novice a mile away by how they comport themselves in certain situations and with people. Guess what — your people can too. You need to clearly develop good leadership behaviors with positive engagement, enthusiasm, energy, and motivation. Unfortunately, gravitas comes with time — if you compare me with my 35 year old me, you will instantly see how cool, calm, and collected I am during critical situations.
Management Tools & Techniques
It's what you know. Most people aren't open to better ways of doing things. "I've done it this way for years" is the death knell of any job or business because something better or someone faster comes along. As with knowledge, you need to re-assess your toolbox of tool and techniques and see if they're still effective and motivating. Bottom line, as a CMO, you need to leverage many motivational, time-management, and process-driven skills to stay at the top and succeed.
If you've been a doer (worker-bee) during your career, you need to ramp up your interpersonal communication skills. I've worked with and seen many C-Level people who are incredible at getting things done, but suck at getting people to help them. When you approach the big positions, it's less doing and more communicating. In fact, it hinges on evangelizing. You need to inspire the people around you and help you achieve your vision for the future. If you aren't communicating effectively, you will lose the troops who will make that happen.
Also, you should be speaking ALL THE TIME. Sample topic: Where will your type of marketing/industry be in 5/10 years?
Connections & Exposure
You should connect with key people in (and outside) of your organization. Connections are the currency of influence and success when you want to become a CMO. Marketing needs the assistance of every other department in your organization, so it's imperative that you develop critical connections to ensure a smooth flow of information, assistance, and resources. The more 'friends' you acquire, the more currency you have to spend when you need to lean on them for a favor.
Get out of your bubble. So many people tend to stay and communicate with the same 10 people regularly. You need to build your connections and get out and meet better tennis players who play better tennis.
I can go SO much deeper with each area - but I wanted to give you a brief intro on what you need to do to move up.