A gigabyte is a measure of infomation [storage]. An hour is a measure of time. The two measure different things and, according to the basic rules of dimensional analysis, conversion from one to the other is not valid. There would be a huge difference between the storage requirements of basic audio and high quality video, for example.
It depends on the quality of the music files.
How many hours of WHAT? For a decent quality MP3, you can estimate 1 megabyte for every minute, and a gigabyte is 1024 megabyte. The storage space required may vary a lot, though, depending on the quality. Movies/videos take up more space, if they are of high quality.
Gigabytes are a unit of storage, not a unit of rate. 10 GB per month = 14.2 MB per hour
16 hours = 0.67 days.
16 hours has 57,600 seconds.