A better question might be how far in advance they schedule the equipment for continuing flights.

It's been a rarity, but lately one of my regular routes has had no plane change. I've been checking every few days and it looks like a regular thing that DL 2437 > DL 674 (GEG-MSP-IND) has had no equipment change.

In the past I've tried for a longer connection, esp last flight of the day, but I'm happy to skip the lounge if there is a reasonable chance of having no equipment change. I assume that with any schedule change all bets are off, but it makes the 51 min MSP connection a little nicer. Obviously there are times when even on the day of the flight they change planes for a variety of reasons, including when the inbound flight is late.