The rule of thumb in one large derivatives group in the mid-90's was "about 3 months" before arbitrage starts causing serious damage. One place to start might be to look for other (anecdotal or survey-based) timeframes like that, see if you can get any sort of curve, trend, or surface out of that raw data.
But I suspect you might find that the valid half-life of any given model has more to do with overall market conditions than with the model itself. I also suspect that you might find a strong correlation between model half-lives and the shape of the yield curve.
Restating as a wild guess: If a model accurately describes some part of the market today, then it will likely do so tomorrow as well, with a probability of P. Assume that there is some relationship between P and the yield curve. For a first approximation, if your model is working with 2-year instruments, then use some factor multiplied by the 2-year yield curve slope to get P, and so on.
Using the yield curve to get P might work better with macroeconomic models, and not so well with micro. There are some obvious cases where this won't work -- if the model depends on some HFT or microstructure feature such as an exchange or counterparty's server load factor, for instance. A half-life rule for micro might be able to use something like VIX as an input though.
Again, this is all wild guesses, I've done little research to see if anyone else has written any papers about this. But a google search for "economic market model half-life yield curve" does look promising.