A property sale usually takes 3 to 6 weeks up to the signing and registration, and living abroad adds whatever the power of attorney takes. If you sign it before a notary in your country, allow 7 to 15 days to have it in Spain with the apostille; if you sign it at the consulate, it depends on how long the appointment takes. If you also need an NIE, add the time it takes to obtain it.
That is why it is best to sign the power of attorney and apply for the NIE before agreeing the date of the deed with the other party.