I don't normally fish waters where I can expect 3 fish a day!
If I did, hmm? Midday - not long enough into a session to make rash decisions! If I'm doing, for example, 3 days/nights, then 24hrs is about right to wait before thinking about moving.
I will of course already have, hopefully, made the right decision swim-wise, after carefully considering swim-availablitiy, visible features, etc, therefore I would show a little more patience, all the time looking for further signs of fish movement!
Obviously, prior to casting a baited hook out, I will have leaded aroubnd a bit and checked out the topography of the lake bed - looking for the usual underwater features, and of course whether there is silt present as this will determine the type of rig I choose.
If there are no further signs of carp topping, rolling, no swirls, leaping, dancing a pirouette (sp), or whatever, then I would normally try something different bait-wise. Such as pop-ups, perhaps off the lead, or perhaps a flouro of some kind! Just something different that might tempt a wary or otherwise disinterested carp!
If I'm fishing 3 rods, and the baited hook is sitting nicely amongst freebies etc then 2 rods would be left alone, and I would dould limit the experimentation to the 3rd rod!
If I've done the above correctly then I won't be changing rigs! Unless I get takes/bites that I have missed, then the rigs would need to be looked at after ascertaining the possible reasons for missing them?
Think thats about it - sorry, probably reads a bit disjointed. Just thinking of me feet, sorry arse!