1-3 degrees. So not much, barely noticable. Your timing in advance of TDC at idle should be around 14 degrees if I remember correctly. The higher your timing, the higher octane fuel and thus harder-to-ignite yet hotter burn your fuel should be (read 91/93 octane). Anyways, advancing your base timing without the spout connector in will advance the overall curve of the advance that the computer gives the engine throughout the power band. At idle, the ECU doesn't advance the timing much because there is no need to. It's not until your engine rpms increase that a spark ahead of near-tdc is needed. There are ways to increase advance earlier in your RPM range (1500-3500) and back it off the normal advance curve later in the range (5500+ rpms) but that requires either a chip or an aftermarket component.
For a relatively stock car, just pull the plug, set the timing at 14 degrees, put the plug back in, run medium or high octane fuel and call it a day.
It'll change more than 1-3 degrees depending on many different variables. IIRC, the 5.0 handbook says the computer CAN advance the timing up to 28 degrees over initial timing...
Anyways, I can hook my scanner up to my '92 5.0 Bronco, give it gas, and see from the scanner that the computer advances the timing up to like 27 degrees. I forget what my initial timing is set at, however. But I know it's no 24 degrees, I think I have it at like 12.
88LX, I was refering to how much the timing would change with the plug being put back in at idle only. Sorry if that was confusing.
baggerman, the difference between 10 and 12 and 14 is minimal, but every lil bit helps!