For my newer layout in my tank (see journals), I had about 3-4 weeks of crypt melt over the tank. Each plant deciding to melt at a different time. Although now they've stabilised and new growth is sprouting through.
My tip would be for anyone growing crypts would be soon as they see a leaf or the steam to the leaf getting in back condition, remove it at the substrate immediately. If left it'll just rot away and you'll have to siphon it out at a later date.
Best thing like with all plants, is to remove any bad conditioned leaves before planting as it'll help in the long run.
Just as a measure of how sensitive they are. I recently added more flow into my tank via a Koralia1 and moved my filter intake at the same time.
This is the only change I have made to the tank and I noticed over the past few days that some of the crypts have started to melt. I can only assume that is down to the difference in flow they are experiencing now or better distribution of the CO2/nutrients within the tank