Oh and yes, it does vary with extension.
Plus I've also seen the shell stick around for longer before disappearing. Although not having any CPU usage so could be Windows getting around to zapping it after it's done.
Here's a delay example (consecutive lines in ProcMon):
"18:24:47.6325859","ZSHELL64.EXE","12892","RegCloseKey","HKCR\WOW6432Node\CLSID\{3C5E2B20-B911-44E2-A2DD-9F05E7B5E775}","SUCCESS",""
"18:25:17.1963760","ZSHELL64.EXE","12892","Thread Exit","","SUCCESS","Thread ID: 39868, User Time: 0.0000000, Kernel Time: 0.0156250"