How Long?

When I finally got the '48 out of its rebirth, the speedometer didn't work right, and after a bit - quit working at all. I pulled the cable out of the housing and just put getting the speedometer right as the last oh-by-the-way project that I would finish up.

Over the winter I sent the speedometer back for a checkout and the verdict was that it was OK. Barring an issue with the drive gear in the transmission, that leaves the cable as the biggest suspect.

You cut most auto store cables to length. Since things didn't work the first or second time - just how much cable should stick out of the housing to engage the drive socket on the speedometer? Any hints on getting it right the third time?

As a general rule, you don't typically want the cable to bottom out at either the transmission nor the speedometer. A good 3/4" to an inch should get you there.
On Ford industrial/farm tractors with a tach, when we would replace the drive cable, you would engage the drive to the governor, bottom out the litter housing without the but, and check for the inner cable to ensure it didn't bottom out. If the cable did, you would cut off an 1/8" of clearance to ensure that it didn't bottom out. The tach cables were a fairly straight shot to the tach, so there wasn't any tight bends for the drive to get pinched in.
On my 2A, the speedometer would bounce from zero to what the road speed was. This went on till recently, then nothing registering at all. I took the cable off at the speedometer, and moved the Jeep a few feet forward, and then back while watching the drive cable. It turned without any play either way. I then took a spare cable and slaved it in to the speedometer. Then I took a drill and put it on the other end. The speedo registered without bouncing. When swapping out the original cable, it was noted that the drive cable was unraveling at the center of the run. That is where the bouncing came from. After replacing the original with a salvaged 70 plus year old spare, everything was good.... For a while. The last run to town we made, the speedo didn't register again. A new cable is on the wish list, but I do have one other salvaged one that can be used in the meantime.