You don't describe what your game is, so it is hard to know how to answer your question about how long it takes and how much it will cost. Just compare the art requirements for a 1-person, but totally amazing game like tiny wings to some mega game produced by the big studios.
Rovio started out making mobile multiplayer RPGs in 2003 and grew to 100+ employees before market conditions forced them to scale back and switch to to smaller casual games (12 employees when they made angry birds, about 50 now). Huge difference in cost between the two. Took them 8 months while working on other contract projects to finish angry birds. (HP Startup Case Study: Rovio Mobile Ltd
Also, since artists are individuals and work at different speeds, it is often the case that employers want a fixed price for the job rather than paying an hourly rate.
That was a really long way of saying that it would help if you point to another game that you think yours is close to, and then you might get some estimates of the time/effort/cost involved.
In the meantime, have you made your game with placeholder graphics, and is it fun to play? If not, it may be early days to worry about where the art is coming from. If it is already shaping into a great game, you will find it much easier to attract a team and might even go to some place like kickstarter and get some contributions to help pay for awesome art using your working demo as a teaser (either video or actual play.)kittiewan2012-09-07 15:40:23