Great question! And it was literally just addressed in this podcast episode with David Skok: http://www.hubspot.com/podcast

Disclaimer the information below (until the last paragraph) is about venture-backed companies.

Short answer: It's dependent on the stage. Less risk = less equity.

Long answer: Usually co-founders (if one is a CMO) would expect around 10%.

If the company has raised seed funding, and you're joining post-money coming in, a lot of the risk has been removed and the company has been valued at a higher level. That phase is less black and white. For a CMO in that range, it's about 1.5%. If you bring tons of experience, you could get as high as 2%.

As the company grows and raises money at higher prices, even more of the risk has been removed and the company is being valued much more than previously. Thus, the stock you would receive is worth much more than it would be at the early stage, which has more risk. The amount will drop down, if it's near the company going public, it could be around the .5% - .7% range.

If it's not venture-backed, David says that typically the CMO is underpaid. If this is the case, then help them think which is more important to the cofounders (who own the majority) - "would they rather have 50% of a grape, or 10% of a watermelon."

I hope this helps. Best of luck!