Probably 10 to 15 hours, but there's so many different things that can affect it. Best to find an artist who's work you like and has a good reputation, go for a consult and go with their estimate. Don't price shop. It's tacky and never results in the best quality tattoo.
The "design" process is a collaboration between you and the artist. If you've done your homework/research then you'll have proof from the artists previous work that their design, layout and use of space is already spot on. Aging well with black/grey realism is a toughie. There are plenty who specialize in that field but whose tattoos (or parts thereof) end up fading and become unrecognizable after just a few years. Part of your research will be to get view or pictures of that artists previous and aged work.
Welcome, and congratulation on coming here BEFORE you jump in.
Generally, the 1st steps are to narrow down the style of art you want and finding what artists nail the style you're interesting in wearing forever.
Understand that quality work is going to cost you good money.
Send us a couple of links to the artist's work you are considering.