"nipple" is just a buzzword used to describe a pipe or conduit run that is short. Nothing says conduit runs can only be 24".
The only 24" related rule I can think of is that If the conduit run is <=24", then "thermal derate" owing to numbers of circuits in the conduit does not apply. i.e. 310.15(B)(3)(a).
However, that rule already allows:
- up to 4 circuits in a conduit if all are 15-20A.
- up to 3 circuits in a conduit if all are <=40A
It doesn't say that explicitly, this must be derived from 334.80, 310.15(B)(16) and 240.4(D).
So the issue doesn't come up much, except with heavy feeder (or dual EV circuits which you should not be doing anyway, ask us why).
But if your intention is to install a large number of circuits, then yes, keep that conduit <2' long between junction boxes.
As far as curves, that doesn't matter. That's not mentioned in the rule.
However, remember the rule with conduit: All access covers must be accessible forever without tools or damaging the building finish, and all conduit pipe must be built out complete before ANY wires are added in. Thus, conduit must be built to be pullable. Corners must either be sweeps, certain listed elbows right at the end, or conduit bodies whose lids must remain accessible forever.