I think the duration will be no more than 1-2 episodes, simply because drawing it out past then might be too prolonged. Yes, walking 10 miles in your enemy’s shoes is better than a mile, but even just that single mile might be enough to convict Regina. If they drag that plot device out too much, then it might start to lose its punch actually. Keeping Regina tethered to Snow White for a prolonged period would basically weaken Regina and force her to give in. So, when it comes time for Regina to choose redemption, we wouldn’t know whether it was something she actually wanted or whether it is basically forced upon her.
Granted, but is 1-2 episodes really enough time for any message of redemption to really sink into Regina? For it to be effective it has to be long enough for Regina to want to change. She caved to Cora in about 5 mins and evil is easier than goodness. So it has to be long enough for Regina to want to be become good and I don’t know if one episode could accomplish that. It might just annoy her to be that close to Snow and had fuel to the flames.