FYI, to answer my own question: it adds 9-months.
I added the code. I realised he actually gave me FOUR codes he bought from a third-party supplier, so I got 9-months x4, thus extension = 3-years!
NB. 5-years is the maximum you can have on your account. So don't add the code until you reach just under 4yrs+3mths, otherwise they will not work.
See info here, under the tab "I have a free offer that came with my computer" – the first bullet point is the key answer:
" I have a free offer that came with my computer:
You can use the free offer in any of these ways:
Redeem the offer now and choose to stay with Microsoft 365 Family. This will add an additional 9 months to your subscription.
Redeem the offer now and switch your Microsoft 365 Family subscription to Microsoft 365 Personal. This will add an additional 12 months to your subscription.
Redeem the offer now using a different Microsoft account.
If you want to stay with your current subscription for now, you can wait until your subscription expires and then redeem the free offer for Microsoft 365 Personal.
If you have an existing Microsoft 365 Family subscription and redeem the offer for a free year of Microsoft 365 Personal, you’ll be asked if you want to convert your existing subscription to Microsoft 365 Personal or stay with Microsoft 365 Family. If you convert to Microsoft 365 Personal, you won’t be able to share your subscription.
To learn how to redeem the free offer and add it to your current subscription, see Switch from Microsoft 365 Home using a product key. "
See screenshot:
Hope this helps!
