I think it depends what you mean by "getting good." If you want to learn to play one piece well, I think you could accomplish that in 90 minutes of work per day.
If you want to reach the level of a professional pianist, that's really hard. Like others have said, it's about efficient practice rather than mindlessly putting hourse in, though, so that's important to keep in mind. Personally I think that 3 or 4 hours per day, at least 6 days per week, is necessary, for 4 or 5 years, so long as those hours are focussed and efficient.
There is research to indicate that our attention stays most focussed for the first 20-25 minutes of practice, so the common recommendation is to divide practice into chunks of that length before taking a short (i.e. 5-10 minute) break and continuing. It's also necessary to be learning about music outside of practice time to inform your practice; so, reading research on performance practice, learning about the music you are playing, analyzing it, etc.