I always suggest to do charge hourly for your work.
But instead of literally charging by the hour, do a couple of drawings of a certain shots (Head shot, mid shot, fully body shot) and time how long it takes you to do them. If it takes you on average 2 hours to do a head shot, charge for 2 hours of work, even if you go under or over.
This will give you good time management and help improve your speed and hopefully retain your skill.
As for how much, find your countries minimum wage or livable wage. Never go under that. You can always increase the amount when you get better, but going below just cheapens everyone else's work and devalues your own.
For me being an Australian with a min wage of $19 an hour, lets just round it to $20 for math reasons.
A head shot that takes 2 hours will be $40. Is it worth it? Maybe, maybe not. But it's better then working for 2 hours with a pay of $10 ($5 an hour doesn't seem that fun).
As for if your are ready or not, it doesn't really matter. Make a pay sheet up of your different prices, and spread it around on commission forums and see what you get. If no one thinks your ready, just keep practicing in the mean time. Don't get discouraged, just focus on improving. You'll get commissions eventually. Just don't stop drawing.
You will get more consistent the more you draw and your style will begin to form.
If you are afraid that it's going to take too long to draw these for the hourly rate, I'd suggest trying to cut your time down. Again, this is literally just practicing what you want to draw, but it it is taking a ridiculous amount of time to get art out, you might need to work on your technique.
