300,000,000 bytes * 8 bits = 2,400,000,000 bits.
2,400,000,000 / 128,000 = 18,750 secs = 312 minutes.
Now this is somewhat of an oversimplification. TCP has some limitations that can slow down the transfer in certain circumstances. But it gives you a general idea of how long to expect. I would add 10 or 20% in a real-world environment.