Assuming 1,500,000,000,000 bytes in 1.5TB (worse case from ISP)
Assuming 25 Mbps video (constant bit rate)
1,500,000,000,000 / (25 * 1,000,000 / 8) =
1,500,000,000,000 / (3125000) =
480,000 seconds = At least 133 hours and 20 minutes
I can say this somewhat safely as the changes that you're watching video encoded at ANYTHING HIGHER THAN 25Mbps CONSTANT BIT RATE is pretty darn low.
(caps used to emphasize, not to shout)
If this is a 'per month figure' bear in mind that a month is about 730.5 hours
The rest of the math should be simple enough.