Jef - how long is a piece of string!?!?!
Personally (and I am sure there will be many more experienced observations than mine) it will depend on the type of water as well as the mix.
For a more highly stocked (nee commercial) water I would tend to go for any of the good commercial mixes. I've been using Sillybait Black with corn, hemp etc added, and its been great, but frankly I think its been more down to the discipline of picking a spot and constantly building up the area by re-casting every 10-20 minutes, rather than the actual mix.
However on my other water, which has a smaller number of larger carp, I tend to use pellets moulded around a much bigger method feeder and leaving it longer (few hours) and then re-casting to different areas.
My only variation on this is where its highly stocked, but quite deep, such as Clattercote. Here I used the pellets because I wanted to know the mix got down to the bottom without breaking up and spreading all over the place. But there again that was only my interpretation!
If you search the articles I am sure Rik, or one of the other proper carpers, has written some pieces on it.