I have bipolar and havr had psychotic depression a couple of times. Most recently (Nov 2015) I could smell myself rotting and believed I was evil and had to die. The symptoms very much reflected the depressed moodvery morbid and deeply suicidql. I think that's an important part of itthe psychosis manifests in ways that are part of the depressed thinking/feeling. I was vaguely aware at some points that these thoughts weren't right, but couldn't fully rationalise that it wasn't real.
I was in hospital for 3 or 4 weeks (voluntarily). Once my meds were increased, I started to come out of it--within 48 hours, although it took much longer for the psychotic symptoms to go and even longer for the depression to lift.
I can only speak for myslef, but I separate myself from my family because I needed to keep them away from the 'badness' in me. It out of my need to protect them that I felt had to leave.
I tend to come out of these epaiodes quite quickly because I know the warning signs (as does my husband, CPN and psychiatrist) so things tend to be put into place quickly and the meds work. If this is a first episode for your relative, I would imagine it would take much longer. Finding the right medication really is trial and error.
Is your relative sectioned? If so, presumably they have a nearest relative who is part of the detention? Either way, they should have a named nurse. I know my local ward has someone who helps relatives.