Commissions Pricing and Example Sheet

I hope this isn't impolite, but may I ask how long it might take you to do a Sketch portrait, and approx. how long for a Flat or Color full? I'm thinking about starting to offer commissions, and am curious how much time an artist of your caliber would spend on these. I feel a bit apprehensive, because I'm a pretty slow drawer. I completely understand if you prefer not to share this information though!!