Paul--
No, I keep the concentration the same and just alter the time, shorter for chicken pieces or pork tenderloins or chops, say, (shorter still for b/s breasts, shrimp, fish), longer for thick roasts and whole birds. If you find a concentration you like (I use a scant 1/4 c Morton kosher to 1 quart water or water-other liquid combo for meats and fowl, 3 T for shrimp and fish) then it is a matter of determining timing.
Cook's Illustrated has two brining concentrations, each with a different sugar amount, but except for decreasing the sugar in brines meant for high-heat cooking (to reduce the chances of burning) I see no reason to alter the salt concentration (except for shrimp and fish as noted) and they do not adequately explain their rationale, typical for CI, except to say that to maintain 'balance' they reduce the salt in brines where they reduce sugar. This is nonsense.
As long as the salt concentration is not over the top, brining time can be extended somewhat without a problem; not that there is a reason to, but if you've spaced out retrieving the meat from the brine and it stays in longer than planned there is rarely an issue if your original concentration was on the lower side. This does not apply to shrimp or fish, however, where timing is better closely monitored. Higher concentrations can leave you with salty meat if the timing goes too long which is why I recommend finding a lower level that works for you. It should be noted that, if necessary or desired, you can increase the salt concentration (even doubling the salt) and brine for a shorter period (scrupulously rinsing after removal from the brine) but I avoid this.
Notwithstanding CI's contention that one shouldn't brine for longer than 8 hours, additional time is usually required for flavor-brining where water is mixed with a high proportion of fruit juice(s) and herbs, spices and aromatics. I don't bother flavor-brining thinner items (with one exception--salmon or other fish I am hot-smoking), I marinate instead as marinades are more suited to relatively thinner meats. I will, however, sometimes make the marinades sort of brine-marinades by upping the salt level. This helps get flavors into the meat but it is then important to remove the marinade completely (by rinsing even) before cooking so that one doesn't end up with a salty finish.
With the exception of a few references to flavor-brines posted here (somewhere) I've never codified my brines, mostly because I wing it at the time. I don't flavor-brine all that much; when I think of all the meats I cook where I've brined, far more often than not I used a straight salt brine or perhaps one with salt and just a little sugar. I always flavor-brine fresh hams and almost always do so for whole pork loins, however. I make a brine similar to Chris's apple brine for smoked turkey (a little more salt, a lot less sugar, a couple more aromatics) but for non-smoked I use a straight brine. (You'll note that in his recipe the salt concentration is a scant 2.5 T Morton/qt water, a workable level.)