Repairing a Relationship

I think it takes quite some time - several days at least, probably more than a week (ETA: I play on long, maybe it's shorter for other life spans but I don't think so). Also, it doesn't turn pink, it's just empty at some point. From there (or even if it's still a tiny bit in the red) you can try to rebuild your Sims' romance again. What may be helpful there is to let them sit in a room with a lot of flirty furniture/paintings with the emotional aura switched on - this will give your Sims a flirty or very flirty mood, making them more receptive in terms of romance interactions. I always use that trick when I want Sims who have been friends for ages to become romantically involved, if they're not flirty they tend to reject the third time flirting in a row, they're like: 'No, we're just friends...'. If they are flirty they are more willing, also unlocks more romance interactions.