The goal is to sufficiently damp down any reflections from bouncing off the backside of the woofer[s] diaphragm[s], hopefully without too much loss of acoustic efficiency, so the amount will vary depending on the material used, ergo unless response measurements are used, the typical DIYer damps by ear and only the woofer[s] should be connected, i.e. disconnect any horn[s], etc. from the test signal.

Altec's rule-of-thumb for a vented alignment was to line only one of each parallel wall [none on the baffle or down in the vent area], so the top and one side, back down to the top of the vent area with either 1" or 2" thick acoustic fiberglass insulation [O.C. 703 'duct board'] depending on how big the cab was and only add more thickness and/or the other side wall if required; whereas many folks prefer to line both side walls initially and call it 'sufficent'.

IOW, once the 'hollowness' [under-damped] is sufficiently damped down, this is typically all that's required and any more begins to over-damp both the woofer's HF BW as well as rolling off the bass [over-damped], which in some rooms/locations and/or personal preference is desirable, though normally better to damp just the vent as required [or even seal it off in extreme cases] as it just impacts its [mid] bass output.

Nowadays, a lot of folks have issues with using FG, with nothing being close WRT its damping properties AFAIK, though in recent years they've been happy with the inexpensive shredded cotton [jeans] as a'good enough' substitute.

GM