I usually mask only 1 to 5 layers and thats my llimit X_x...thats when i decide that its not worth it.
I once made an epic amv witch chroma key.I was/am really prod of it but it god deleted cos my pc broke down + when i started to render,it took like 3 hours to do 15 % and it just crashed and broke my audio system in windows or screw up my codecs and i was unable to view videos or listen to music >_>