This is a tough one. For self-bows, or any all-wood bows, there is never any question - do not leave strung any longer than absolutely necessary. For modern bows with fiberglass on back and belly, I am of the opinion that it isn't an issue. Certainly, there are some bows that are under an extreme amount of tension when only braced, such as my '53 Bear static-tip Kodiak. That bow has the straight limbs three inches forward of the handle when unbraced, and then it has the recurved tips added to it to boot! There is a lot of tension on that bow when braced, so no way will I leave it strung too long. My Samick Red Stag, on the other hand, has a deflex design, and the limbs are not under much tension when strung, and I have left it strung for years, and recent testing shows absolutely no loss of draw weight. I would also say that modern-materials longbows that are of straight-limb design or very mild reflex/deflex are probably OK to leave strung, due to the limbs bending mostly throughout the entire limb, therefore not stressing any one area too much. Long and short of it - weekend, no sweat, more than that, make your own decisions.