Are you referring to memory or disk space? Nevertheless, since video is not saved to a file or memory, consumption of memory or disk space does not grow with session lenght. To check memory usage you can use Windows Task Manager.
Skype video calls must use some megabytes or gigabytes.
How does one work out how many MB or GB it will be for a video call session of:
15, 20, 30 or 40 minutes.
Assuming Skype uses same 500 kilobits per second for both uplink and downlink video, then the numbers are:
10 min session: 300 megabits = 37,5 megabytes for both uplink and downlink
20 min session: 600 megabits = 75 megabytes for both uplink and downlink
30 min session: 900 megabits = 112,5 megabytes for both uplink and downlink
So even on a ADSL1/maximum 1500 kbps or ADSL2 (with more/higher kbps) from an ISP,
a high quality video call on Skype or perhaps facebook too, would use only 500 kbps?
That 500 kbps was your assumption. You can see the real bandwidth (both uplink / downlink) used from Skype call technical info window. Or alternatively you can use some third party internet traffic monitor applications.
I mean to load up and use/be/stay connected during a video call.
Be it for a duration of for instance either: 15, 20, 30, 45 or 60 minutes.
What do you mean by "on call"?
(Residential anyway) broadband/ADSL internet accounts have a limited data allowance per month, that can vary from 10 GB, 20 GB, to 50 GB per month.
I would want to know, how much of that data (megabytes or gigabytes) would a video call session on Skype or even facebook's equivilant video call program use, say per minute, or even per 15 minutes?