I peeled and sliced peaches last night and combined them with sugar and lemon juice. How long can I keep the fruit in the fridge before I cook it down into preserves?
I keep my fruit+sugar at room temperature, covered, for almost a whole day before cooking it down for jam (~18hr, maybe even longer); if yours is in the fridge, I'd say a couple days is ok for sure and beyond that just taste and see. Some oxidation will go away with heat, so any brown bits shouldn't be too big a concern.
I should add that I never cook down jam withOUT letting the fruit sit in the sugar for a while first. I find the fruit juices are drawn out better and the flesh breaks down better that way.
I've gone as long as a few days with no problems whatsoever. In my opinion, the longer the better. You'll have a lot more juice than if you just let them sit overnight, so pour it off into your jamming kettle and reduce it way down, to a thick syrup, before adding the peach slices. (I do that with any preserve, even if the fruit has just macerated overnight. It results in a much tastier finished product.) ;o)
I agree that I like to macerate the fruit prior to making jam. I will often time jam making to coincide with rainy weather, so the fruit and sugar sits in the refrigerator for a few days waiting for the weather to "cooperate"!
We always macerate our fruit for at least 24 hours and as much as 3 days for blueberries. We never use petin and the maceration helps extract the natural pectin in the fruits to give us a perfect set every time. By accident, we had a batch of raspberries macerate for about a week before jamming. The fruit was starting to ferment and we ended up with the best batch of jam we ever made.