So you're talking about a 1/2-13 x 1/2" long threaded insert? which accept a 5/16-20 bolt or somethign like that.
I just got some takedown limb hardware from bingham's and they use 1/2-13 x 3/4" long IIRC.
My guess is its overkill either way.
Here's my take on it:
Longer limb = more stress on bolt
Shorter distance from the bolt to the end of the limb pad = more stress on bolt
(both have to do with leverage)
If you're doing a rear mount limb then this is the distance from the bolt to the base of the limb pad.
Higher draw weight = more stress on bolt
I wouldn't worry unless you were making a really long bow, with a short riser, and high draw weight, and you plan on putting the bolt in a poor location.
I once used the cheap cast metal inserts from HD which are clearly not good quality (they are made for furniture or something) in a takedown bow, maybe 40-45# with 27" limbs. I think these only take a 1/4" bolt. Haven't gotten hit in the noggin yet, not that I recommend this at all, ever.