When Can You Start Charging?

From my personal experience you can start charging when people are ready to pay. Essentially before you start charging, you will need to take many factors into consideration. You will of course want to make sure you have a versatile and strong portfolio with images that best display your look and style, as well as what you can bring to the table. You will likely need to put fourth an insane amount of hard work to get to the point where you are giving out rates.

Image by: Wiz Of Oz Photography
www.wizofozphotography.com

Another thing to consider, is when being offered a shoot by a photographer, after you have already built a substantial portfolio and have had the experience, these are some things you will want to take into consideration before sending your rates to the photographer in question: Is his work equal quality or better quality to your portfolio? If so, you may NOT want to offer him/her your rates, as they can benefit your portfolio and get you exposure. If the photographer has an empty, outdated, or not great quality images, I feel it is acceptable to send your rates. 

Photographer: Paul Parks of Blatant Pixels

Rates should vary on experience and market. Do some research to see what other models in similar genres are charging, and meet in the middle. 

Getting paid right away is best accomplished NOT by being a freelance model, but by signing with a commercial agency, or model 18+ work. 

Another great way to receive payment(non monatary) is to suggest that you will shoot for tear sheets or web tears. Being published is what it's all about, whether it's for a company, product, service, the more published you are the more offers to be published you will see.