The Tax Office has 6 months counting from the closing date of the RENTA income tax campaign, to pay out any refunds you are entitled to, according to your RENTA declaration.
For 2018 (declaration 2017) that will be the 2nd of January 2019, as the RENTA campaign closed on the 2nd of July this year.
Refunds are handled manually, there is no automatic system, so some, mostly small and straightforward ones, will be paid out quickly and others will be postponed till the end of the window.
If the amount due is not returned to your bank account within the established window, you will be entitled to interest, and this will be added to the refund automatically. For the RENTA 2017 (filed in 2018) the interest percentage has been set at 3,75% in the General Budget for this year.
Meanwhile, you can check the progress of your devolution online. Explained on the following link.