Depends entirely on content!
If it's a simple headshot, sketch-to-finished-colours would take me maybe 15-20 minutes? If it's a more complex panel with lots of detail in the environment or tricky perspective and lots of foreshortening, it might be upwards of an hour, an hour and a half.
... It's hard for me to estimate. I mean, I don't work on my comics panel by panel. I do it stage by stage; sketch the entire page, ink the entire page, do flats for the entire page, etc. Time spent finishing a whole page varies from a low of 3 hours for super-simple pages to a high of 12 hours for super-complex, crowded pages.