The Enterprise D's maximum speed is warp 9.8. This is equivalent to around 1,015,313,576,887mph
Given that Mars is typically around 249140,000,000 miles from the Earth, that means that the Enterprise could (if they were monumentally stupid enough to do an in-system warp jump) reach Mars in substantially less than one second.
That all being said, the fastest we ever see a ship traveling at sublight speeds is half-impulse (half the speed of light). That means that a realistic journey would be around 10 minutes, depending on the amount of time it takes to get your luggage through security.