About four months ago I bottled/kegged 50 litres of a pLambic that I had fermenting for about two years. I used 60% raw wheat 40% pils, about 150g of some very old fuggles and the Wyeast lambic blend. I was quite happy with the brett. funk that I got out of it, but it could have done with a
lot more sourness. Part of it was kegged as a straight unblended pLambic, and the other half was distrubuted to roughly 10 litres of Cassis and 20 litres of Kriek. I used concentrates for the Cassis (blackcurrents being fairly hard to find) and was - as expected - pretty dissapointed with the result. I found the amount of sugars in the concentrate brought the alcohol up to quite a noticable level - fairly hot and quite out of style. I used jars of morello cherries acquired at the local supermarket for the Kriek, but apparently nowhere near enough. After four months on the cherries (about four and a half kilos) the resulting brew is a lovely deep red, with very little cherry flavour.
Things I would do differently next time would include not relying on the Lambic Blend as the source of yeast/bacteria, instead utilising the individual cultures one can buy and pitching them at around the time they typically become active in the brew. A fairly sterile approach to what is supposed to be a spontaneously fermented brew, but I would personally rather brew a nice example of the end result than spend years relying once again on chance.
I fermented this for roughly three weeks in plastic before transfering it to a 55 litre keg that I converted specially for the purpose. I believe this may have removed a lot of the bacteriae lying dormant in the sediment layer so I would refrain from racking pLambics in the future and stick solely with the stainless fermenter, unless someone who has actually been to Belgium and spoken to the brewers there would recommend otherwise. I would also place a single french oak stave inside the fermenter... something noticably lacking in my first attempt.
As for the fruit additions; I will never, ever use concentrates again, nor skimp on the fruit additions. From my experience I would say that Malnourished's example of 2kg/G is a nice ballpark figure. However, I would likely head towards the 3kg/G mark, leaving some straight pLambic aside for blending later if you have gone over the mark (at the very least, you will have some straight pLambic to drink if you don't overdo the fruit... never a bad thing
)
Finally, one thing I have been contemplating is building a solero type system of the large 55L kegs. I would begin a batch every six months or so, topping up the kegs as I go and kegging/bottling/racking-onto-fruit 25 litres from the bottom keg each time I transfer a fresh batch into the top keg (if that even makes sense). A fair bit of effort, true, but it may well be worth it in the end.
