It's hard to quantify it because different things use different amounts.
A full web page is usually around 1mb. An hour of youtube watching could take you up to around 100mb
Most phones have a data counter facility.
you won't actually be charged if you do go over the 100mb limit. you will just recieve an email from giffgaff warning that you will be cut off if you continue to use that much data. If that happens then its best to buy a goodybag which is unlimited.