You may not have to do the scent swap phase for very long at all. It will depend on how reactive your cats are and how serious the aggression between them has been in the past.
The Next Step with Mealtime
Now it’s time to open the sanctuary room door a bit when feeding the cats. Have the cats eat within sight of each other but far enough apart so nobody feels threatened. Keep these feeding sessions brief by offering a small amount of food. It’ll be more productive to do brief sessions that ends on a positive note rather than attempting a lengthy training session where you risk pushing one or both cats’ tolerance limits. If one cat routinely attempts to charge the other, use a door stop to prevent the door from fully opening. You can also place a hook-and-eye closure on the door temporarily.
Wide Open Door
Move to this phase when you feel the cats are comfortable with the previous one. Don’t rush the reintroduction process. If you’re in doubt about whether it’s time to move on, stay at the current stage a bit longer. There’s no time schedule here. What matters most is that you want the end result to be that the cats return to a friendly (or at least neutral) relationship.
If the thought of the fully opened sanctuary room door is scaring you because you’re afraid one cat is going to charge, try an interim step of stacking two or three baby gates across the entrance. Another option is to install a temporary screen door (with secure pet screening). The cats will be able to see each other but won’t be able to engage in a physical fight. If you choose the interim step, make sure you close the actual sanctuary room door again when the feeding session is done. You can even use just one baby gate during the feeding sessions as long as you’re standing by the door in case the worst happens. Even though the cats could easily jump the baby gate if you’re using just one, it may serve as enough of a comfort zone to relax the cats so they’ll be at ease enough to eat.
As you progress with the feeding sessions, gradually increase the exposure time and start to let them wander around more.