I haven't paid either IBI or non-residents tax. I completed in March 2015
I emailed my lawyer to ask when I would be expected to pay and what costs there would be as an owner who doesn't rent out my apartment. The response is below.
The IBi will come our around September, we will email you once it is ready and inform you in order to pay it and set the direct debit, . I think that it could be around 250 €.
We will also email you around September to inform you about the NRT but aprox 60 €.
So, much better than I expected