How long should glow plugs stay on

Some morning my car takes 3-5 seconds to start when turning the key. When it takes longer shoots out unburnt deisel. Glow plugs are on for i would say 1 second. Parked in a garage only happens in the morning. No codes 2011 jetta tdi. Should the glow plugs be staying on longer and maybe i have a bad glow plug?