I wonder how the efficiency of a diclofenac patch decreases over time. E.g. if a patient wears a a diclofenac patch for 24 hours, how useful are the last 12 hours. Ideally I would like to see a curve (abscissa: time; ordinate: efficiency). Assume that the diclofenac patch is used to treat a lateral epicondyle of the humerus.
As previously said, this is dependent on the product. However, there is at least one study available comparing the bioavailability (which is what you are asking about - in this case, how much diclofenac permeates into the skin) of several of these patches and the differences don't look too big to me.
Patel, Kunal N., Hetal K. Patel, and Vishnu A. Patel. "Formulation and characterization of drug in adhesive transdermal patches of diclofenac acid." Int. J. Pharm. Pharm. Sci 4.1 (2012): 296-299.
What you are interested in is table 2:
As you can see, they do look rather similar, and an obvious feature is that the bioavailability for the first hours isn't as "good" as later. So in my nonexpert opinion, keeping a 24 hour patch on for 24 hours would be preferable to switching after 12.
Basically this depends of the product you use. Your question does not give enough information to provide a "curve", as this has many variants including the produc you use, skin temperature(!), humidity and other influences. Read product instructions. These patches are generally made to provide a more or less constant uptake of the used drug, so in a perfect world with given temperature, humidity and skin type it should be more or less constant for some hours. The amount of hours depend on the specific product.
As you seem to assume right the effect decreases after some time, so if you change every 12 hours you may get more effect than every 24 hours, but most producs seem to be made to work for 24 hours.
In conclusion, if you use a 24-hour patch it won't make much difference to apply it every 12h, because the drug intake/hour should be more or less constant. If you leave your 24h patch for a longer time, like 72h, it is to be expected that the effect decreases.
http://www.accessdata.fda.gov/drugsatfda_docs/label/2011/021234s005lbl.pdf gives you some information, but better you search for the product you use.