It's going to depend on base and major partner more than regional airline.

The length of trips really isn't up to the regionals, practically speaking. They get assigned flying from the majors, and much of that flying includes the last evening flight to the outstation, and the first flight morning flight to the hub.

Both of those flights are each going to involve a RON crew, unless it's a standup/highspeed which will require one crew. The number of outstations served by a hub, and which flights mainline covers, is going to determine the need for RONs...the regional then has to build trips which cover all of the RON's. Regionals would vastly prefer to NOT buy you a hotel and to NOT pay your per diem and trip-rig while you sleep.

For those of you who don't understand why there aren't more locals, it's because Locals cover zero RON's.

Two days have a 50% RON coverage/Duty Day ratio.

Four days have a 75% RON coverage/Duty Day ratio.

Standups have a 200% RON coverage/Duty Day ratio. If mainline's schedule (and the stage length) allows legal standups, they are best RON bang for buck.