Unless you are watching on a 4.5" phone screen, I wouldn't even bother with that low a res. It will be very pixelated. 480p is what is good for old programs from the 70's like MASH that were recorded in (relatively) low resolution and 4:3. Encoding them into 720p would do nothing but increase the file size. If you want to see the difference sideload ShowBox / ShowBoxLite. When you goto view a video you will have 720/480/360 options. Watch a the same bit changing the resolution and you will see the difference.
These are rough figures excluding any overhead and can vary depending on the codec and bit rate the file has:
360p = 250Mb/hr = 100hrs
480p = 400Mb/hr = 60hrs
720p = 900Mb/hr = 28hrs
1080p = 1.5Gb/hr = 16hrs
I only asked because Telkom gives you 25gb free to stream from DSTV now or netflix app when you buy a +5gb data bundle. I just wanted to see how many hours I can get out of it. Using your calculations, it's about 3hrs a day for tho entire month.