For me it usually breaks down like this for my comic
Sketching/drafting usually takes me 30 minutes to an hour depending on how cohesive the page's concept is to me.
Sketchbook to computer transfer takes about 15 minutes because my process isnt efficient and involves emailing myself a bunch,
The actual digital production of a page takes about 6 hours, but I've spend up to 11 hours on a page before. 6 is definitely the average.
This is all gonna vary from person to person however.