The last two answers are spot on. Each type of operation has its own delay time. You get to know these habits after a while. And repeaters usually don't need much if the tail on the repeater is long. Then there are half-duplex frequency pairs that drive me crazy. Those need as little delay as possible, but if you are scanning a lot of channels, you're not likely to get replying transmissions. I wonder if priority settings on those channels would work. I might try that.
On one agency that uses a lot of air to ground callouts with two to four second gaps, I found that four second delay has to be it or the scanner would never hear anything else but that one channel.
But what I do know is that the RR Database with only 2 seconds delay is not nearly enough for most situations. You only get tiny bits and pieces. I can't tell you how many times I heard fire tones go out and it scanned off way before the actual dispatch was announced. And if you're driving a car, 2 seconds isn't enough time to find and press hold! I would like to see 5 seconds the default with exceptions being made for half duplex channels and long tail repeater channels.
Phil