I think this would be a fairer way of operating the gigabags. The product is sold on the basis of the amount of data. presumably GiffGaff would be happy for me to use the amount of data on my gigabag within the specified time. So if I've paid for that amount of data I should be allowed to use it all however long it takes.
I'm sure if I used all my allowance i a week GiffGaff would be more than happy to let me purchase another.
Come on GiffGaff step up and provide a product nobody else is. I'm sure you would have floods of tablet and low usage users flocking to buy your data sims.