It really is up to you. I didn't charge when I was training, but as soon as I passed the NVQ assessment in whatever it was, I started charging.
In hindsight though, I could have charged really! One way of doing it is to ask for a "contribution" towards your products, time, petrol etc and make a suggestion of how much. Never say "Oh it's five pounds to cover the cost of my products" - if people know that your product cost is £5 they might find it hard to swallow if you qualify and then start charging them £20 for the same treatment, iykwim. x