I think it means how long it will take the support to get there when it's needed. I'm guessing it would vary according to conditions. It would take a brigade longer to move up than a company. If there were intervening obstacles (swamp, woods, rocks, steep hills), shorten the distance to keep to a time. Flat ground with good roads would allow the support to stand back further.
Just exactly how much time supporting distance assumes, I wouldn't know, but I've not seen time mentioned, so I'd guess that there was an understood interval (perhaps 10 minutes?) and the supporting commander was to place his troops where they could move out and be on scene at that understood interval.
Just guessing, but it's a great question and I'll look for a better answer from someone.
Ole