Interesting question, indeed...
I'm not sure about a maximum time frame for enhancing the flavor...considering how curing takes place, spread out over long periods of time, the meat should continue to absorbe the flavors of the seasonings, but I would have to consider the possible safety issues with a re-packaged meat. The longer you wait to cook the meat the greater risk for contamination problems, IMO.
I have done 8-12 hours many times in the past. The main thing with rubs which can adversly effect the product when applied a long time prior to cooking is that salt will draw moisture from the meat. More time and more salt draws more moisture. It eventually can turn into a marinade of sorts, simply because of such large quantities of water being present on the surface. Not that there's anything wrong with that, but the rub may not adhere to the meat very well at that stage in the process.
Here's a few thoughts to consider: increased sodium reduces biological risks, but in turn causes the drying issue. My first though was to suggest using a reduced salt blend for a dry rub for anything beyond 12 hours, but then, the bacterial risk comes into play.
It may be a compromise if using a normal salt content, as you will loose quite a bit of rub before the meat ever sees a smoke chamber grate, however, the seasonings will have absorbed more deeply into the meat. My experiences with home-cured/smoked corned beef pastrami indicate that the spices can absorbe very deeply over long periods of time. If in doubt, I'd just use Morton's Tender Quick instead of salt in the dry rub. That would remove any thoughts of biologocal issues from my mind, allowing days upon days for you to get the meat cooked during it's long soak in spices.
In either situation, if you want a really nice bark on the meat, you may want to apply a second coat of dry rub (using a reduced or no-salt rub) immediatley after unwrapping the meat (while still moist) and go straight to the smoker with it...should work out great.
Eric