The word "cure" is used in this thread to refer to two different processes.
First is the hardening of the cement into good strong concrete, meaning the evaporation of water you added to make your mix. People in the concrete business call this "curing." That's where you want to keep your creations wrapped in wet burlap or in a plastic bag for at least 5 days after you make your mix and shape it into your creation. I used sand in plastic containers for my molds and just spray water on the sand once or twice a day. I'll let my creations "cure" (i.e.- harden) from one weekend to the next (7 days) as I don't really have the time to mess with it, except on weekends.
Second is the process of soaking your creations in freshwater in order to leach all the bad stuff out of your creations. What will happen is the portland cement will raise the alkanity (pH) of the water, making it essentially basic, meaning the opposite of acid, but just as corrosive. Change the fresh water on whatever schedule suits you. One of the posts mentioned adding vinegar. Vinegar is acidic. As you learned in high school chemistry, acidic and basic solutions neutralize each other.
Since I'm a patient man, I intend to let my creations "cure" outside in freshwater in big 44 gallon Rubbermaid bins for a good long while, say two months. I will either change the water frequently, or just stick a garden hose in it on a slow trickle to provide both movement and replacement, but since I haven't dug them out of the molds yet to see how they'll turn out .....
Apart from the usual blob shapes with holes, etc, I made two structures that may (or may not) turn out to be interesting. First one was a lattice on legs, very similar to the grid-irons you put in a fireplace to raise the logs off the floor. I am wondering if raising all my rock up off the sand by 2 or 4 inches will just look supercool or give fish a nifty playground underneath the mountain of rock in addition to the benefit of exposing more surface area of my DSB. The other neat creation I made was a four legged "arch", with a "table" at the top, about 20 inches tall with the legs about 15 inches apart. For both of these, I figured strength was more important than porosity, so I used a 3 to 1 ratio. For my blobs, I went with the 7 to 2 and 4 to 1 ratios.