How long a name will stay until it expires?
36000 Blocks. This currently equals to around 200 days.
name_update: error: {"code":-5,"message":"Invalid namecoin address"}
This shows that you made a syntax error. The error message "Invalid namecoin address" is displayed only when you transfer a name to somebody else. This is not what you want.
Check the value field of your name_update command. I think there is a space or a
special character in your value field.