I think it depends upon the cake and the difficulty of the decorations.
Gumpaste items are done ahead of time...torting,icing,final decorating can take me 4 or 5 hours.
OMG! I must be the slowest cake decorator ever. I'm very critical of my own work and will keep working at it until I get it right. If you go to my gallery the pink and green gift box cake took me about 8 hours to decorate and probably another three hours of baking. The snake cake took me about six hours to piece together and decorate. A regular, standard cake takes me about 2 to 3 hours. So far I'm not making a living off this new hobby or I'd have to move back home with the folks.
I'm glad you asked this Pimples25! I kept telling my DH that it takes me way too long to decorate a cake. I recently got into cookie bouquets which I thought might go faster but I spent hours yesterday on one bouquet! What seems to take me so long is deciding on the colors, mixing icing colors, and preparing the bags. The actual putting-icing-on-cake part doesn't take as long as getting ready to put icing on cake!
I told my sister that if I worked in a baker I'd be fired for being too slow. I'd produce like one cake per day! haha
I thought I was the only slow one out there! 
It's the dang'd bags/mixing colors and loading up the bags that gets me bogged down. 
The decorating part isn't too bad. It does depend on the cake, though.
The flag cake w/star work in my gallery took about 45 minutes. The two birthday cakes took about that, too.
The wedding cakes? Shhhhhheeeesh. With smoothing and patterning those--I probably had 5 hours time in each one. The smaller white/red wedding cake didn't take as long--it wasn't as BIG!
--Knox--
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