First look here and set you Netflix accounts to stream in standard or low definition:
https://help.netflix.com/en/node/87Going by what Netflix says the usage is, it is then simple division. The 15 GB is 15 with 9 zeros and 300 MB is 300 with 6 zeros.
15,000,000,000 divided by 300,000,000 (300 MB/hour) equals about 50 hours at low definition. At standard definition (700 MB/hour) it would be about 21 hours of viewing.
This is assuming you do nothing else online though. Plus there would be a small percentage more for upload communication.
Like Chris says, even at low definition 5 hours per day would eat your Anytime data in 10 days.