Just to clarify how CAS generally works for those that aren't aware, based on Afghan (Sharkey you need to read this as you clearly aren't aware how anything works these days - it isn't the Malvinas).
You never know when you will need CAS, unless there is a pre planned op but you can make some good guesses based on historical data (dawn attacks, night raids, avoiding midday heat etc) so you can have more CAS airborne at the right time. In Afghan there is quite a lot of FJ CAS plus Reaper (which actually carries more weapons than GR4 but can't do SOP/SOF or strafe), or even a Pred if you are desperate (only one weapon). These aircraft can be doing other stuff (ISTAR/NTISR for example) or just be turning dead trees to noise (XCAS) waiting for something to happen. These will give rapid response times, certainly in central Helmand which is an ATC nightmare with all the traffic. On that point the biggest factor in getting a weapon on target is often not getting an aircraft eyes on and a 9-line sent but actually clearing the airspace (a plus point for helos).
However there will inevitably be times when there is nothing available airborne so there will be GCAS (ground alert CAS) these will have an alert time, lets call it 10 minutes for sake of argument, but that is a "no longer than". The limiting time here is waiting for kit to align and spool up and the wind up time of the engines(s). Given that Harrier only has one engine then it may gain 30 secs on GR4 but then again the WSO can be doing the kit at the same time as the pilot is doing the engines so I would suggest there is no discernible difference between the 2. Once airborne the limiting factor is obviously distance to run and transit speed. Harrier wouldn't be at too much of a disadvantage given the short distances usually involved in Afghan. Likewise Apaches are rubbish for this if the CAS is 100nm away but great if your AOR is minute (TFH for example).
In Afghan if there is an urgent need for CAS, ie a TiC, then you don't request it through the ATO cycle!!!!! There are other means that will provide what is needed as quickly as possible. I don't know the actual stats (and wouldn't say if I did!) but for Helmand/Kandahar area I would be surprised if it is more than 10 mins in most cases, 30 mins in all cases.
Hope that has helped to clarify.