|Correct me, if I'm wrong, but I guess you meant 50 megabit not 50 megabyte.|
In that case, 50 megabit/second divided by 8 gives you the value in megabyte and is 6,25 megabyte/second.
This is the maximum speed you can get.
But at all internet communications, you need to subtract the so called overhead, which is needed to ensure the correct communication on up- and download including error correction a.s.o.
So if you get 5 megabyte/second (is 40 megabit/second), you got a good speed.
But keep in mind, that the speed also depends on the source, where you are downloading from. You may also experience lower speed on weekends, where a lot more people get the time, to extencively use the internet.