How much?!

I’m installing the latest software updates on my MacBook Air, but I’m wondering how long this is  likely to take.  There seems to be some doubt about how much data needs to be downloaded:

Not a number, apparently

Not a number, apparently

For the less technical readers, NaN isn’t somebody’s grandmother, it’s a programming thingy which indicates that the value being looked at is Not A Number. Which is not what you expect to see when updates are being installed….