The Montaña Blanca circle is roughly 5km and took me about 3 hours with photo ops and water breaks. The initial ascent is quite steep, it starts off as a good dirt road, but the final section is very steep and loose gravel under foot. The descent is a narrow path, a little awkward in places, but no real dramas. I would however recommend a walking pole for the down hill sections. The route is mostly well marked with San Bartolome trail markers and GR-131.
There are striking views throughout this walk. Towards the coast, down into the village of Montaña Blanca and the vineyards of LaGeria.
To read more on the history of the water cisterns in the area, you can check out the following site:
The site is in Spanish, so if it helps I have made an attempt to summarise a few topics of interest in the History section.