On some beaches you are still not allowed to walk dogs, no matter what time of year it is!
In Manliva area for example, dogs are not allowed on the beaches at Sabinillas, La Duquesa and Castillio de la Duquesa, however just up the road is Casares, and on the beachs there you will see a sign saying "Perros No!", but across it is a sign saying during the period 1st May to 30th September you will be fined if you walk your dog on the beach.
Therefore, most dog owners in the area walk there dogs on the beaches during the 'off-season' period, but it is still possible you could get fined if your not careful.
The law is down to the individual municipalities in the Marbella region, so check with your local town hall.
So the law here in Manilva is not at any time, however in Casares you can, but with certain restrictions.