I've tried to set a subscription length to 12 months on the product object, but it doesn't auto populate when you select the product on the quote line editor.
is there a way to auto-populate subscription term on the quote line editor ?
The Subscription Term on the Product and Quote are controlling different things.
On the product, this is saying that a subscription product is priced at X cost for that term. If you have a product with a subscription term of 12 months and quote it on a Quote for 3 years, CPQ needs to take the 12 month price x 3.
It is possible that you could select a product with a 12 month subscription term and then select another product with a 6 month subscription term - you would need logic to decide how you want to handle that.
You could auto-populate the subscription field on Quote with a WF Rule on creation or change the default value of the field, but that isn't dependent on your product. You could also use a Price Rule to set that field - that would run when the quote is saved or the user hits the calculate button.