Does the 250mb mobile data upgrade mean I get an extra 250mb of data for free each month or only for one month?

Free each month for 24 months fro. The reception
You need to apply them and wait the next billing cycle to use them

You get it every month from the first day you got it until 24 months.

So, enjoy it!

It is valid for 24 months starting from the time you received it (whether you apply it right away or not). You also can unlock more than one upgrade slot so you will be able to get even more free data.

Every month

It will be yours every month once activated. However it expires after 24 months but you'll receive something else by then.

yes, every month for 2yrs. Also, it will be roll over to the next month if you haven't use it.