Muriceopsis flavida is photosynthetic, diodogorgia - non-photosynthetic.
Time to decline for Diodogorgia nodulifera: hard to say, I had decline only once, when they were kept for a 1 - 1.5 month in a low flow with once a day feeding. If your flow is good, it may be much longer.
Signs: skin loses glossy appearance, then becomes looking rough. Central core is likely partially dead this time. Photos for red morph of Diodogorgia are
here.
My yellow morph looks skinny all the time, but grows new branches.
saltn00b:
If you wish, take a look at Plexaurella grandiflora, Plexaurella dichotome and Pseudoplexaura at Image Search, if they look like yours. Don't have them, though.