At what point do you need longer front and rear LCA's?
There is never really a point where you "need" longer front and rear lower control arms. When you lift your Jeep, your axles will be pulled closer to the center of your Jeep and that will change your overall wheel base, change your suspension geometry, change your caster and pinion angle. Installing front adjustable lower control arms with a 2.5"-3.5" lift will allow you to add a little caster back and that will help improve handling. If you are installing a new double cardan rear drive shaft, you will need to get rear adjustable UPPER control arms to set your pinion angle but, there is no need for lower arms. Having said all that, IF you wish to compensate for your axles shifting inward, you will want to get BOTH upper and lower arms for BOTH front and rear. The lower arms will allow you to set your axle position and the front uppers will be used to set your caster and the rear uppers will still help you set your pinion angle.
I've read some posts that address the upper control arms being needed around 3.5"
Don't know where you would have read that but that is just inaccurate information. Adjustable rears are NEEDED anytime you install a new double cardan u-joint style drive shaft as they will allow you to set your pinion angle. Adjustable front arms are only really "needed" IF you install adjustable front lower arms and use them to set your axle position forward a bit. At this point, the front upper arms will be used to set your caster angle.
but I haven't seen anything regarding the magic number for lower arms.
There are no "magic numbers" per se. Adjustable front lower arms alone can be used to add back a bit more positive caster after any lift and that will help improve handling at highway speeds - it'll help keep your Jeep from wandering. There really is no need for adjustable rear lower arms.
Do longer lower LCA's improve flex, ride, or what?
NO. Adjustable lower control arms WILL NOT improve flex. Front lower arms can allow you to add back in a bit more positive caster after a lift and that will help improve your handling at highway speeds. Rears are totally unnecessary. Installing lowers and uppers at the same time will allow you to correct your axle positions but the shift from a lift is so slight that it really isn't necessary to do. Installing "longer" lower control arms as in 1" longer or more is just a marketing ploy and stupid. The amount of shift your axles will see is only about 1/2" and extending your arms longer than that will only cause spring bow and potential rubbing issues.
