Duration of Dispensation's procedure

Does anybody know how far back in time from wedding date does one have to look for a "dispensa por consanguineidad"? How long could the process take when it was already known that the bride and groom were related in the fourth degree? Was it a matter of several months or of years? Does anybody have a suggestion on how I should look for it?
Thank you in advance for any help you can provide. Looking for this type of documents is for me "advanced research".
//Belinda