Delay should typically be 2-5 seconds, depending on the average time between transmission on the channel. If you set it too short, the scanner will resume scanning before the other party has a chance to reply. If you set it too long, the scanner will loiter on a dead channel unnecessarily.
Threshold depends on the system you're monitoring. Use Auto unless you have trouble, then you'll need to choose Manual and try different settings until you find what works best.
Hold time should generally be set to zero, unless you are having issues with a system. If so, setting it to 1-2 seconds will force the scanner to loiter on the control channel longer and hopefully miss fewer reply transmissions.