There's quite a few factors on that dimension; belly height, tire size, 4 link geometry, driveshaft plunge, lower link placement on axle, wheelbase, triangulation, single shock or c/o and bypass, etc, etc.
There's also a few trains of thought on what works better in the rocks. Drop them too low, rock magnet; mount them to high without consideration to combat link roll, =expensive damage to shocks.
If you have a moderate belly height (17-20") on 37-42 tires with 112-117 wheelbase, a 40-45" trailing arm can work well with proper geometry. The shock bolt must be below the imaginary line between the rod ends/bushing, and a uniball/leaf spring bushing, or some other type of mechanism must be used to combat link roll due to rocks pushing on the arms, if going below. If going above, you will need to bush the lower shock heims or use a uniball at the forward link mount to combat rolling of the links. Both methods have been proven successful if done correctly.
Is this a new build or modifying a current configuration? More info about rig and usage will help.
Edit: I'm not a pro, but I have done a lot of research to aid in doing trailng arms on my build, that hasn't seen the trail yet. There are many more experienced folks that could chime in. Just sharing info from said research.
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