ow long should the glow plug light stay on?

Off the top of my head, under 2 secs on average. You can adjust it if you have a vag-com cable to be longer or shorter.

Is there a way to make it longer without a vag com cable?

Theodt said:

Is there a way to make it longer without a vag com cable?

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not that I am aware of. If you want to do any serious work on your car or want to get access to diagnostics, etc., a ross tech vag com cable is needed.

PS, the mk4 cars for some reason, tend to have shorter glow plug duration than the mk3 cars. Don't know why. The one thing you can do is to turn the key off and back on to cycle the glow plugs again.

Another thing you may want to consider is the glow plugs may be aging or the harness could be bad. Glow plugs get very hot and do wear out slowly.

clean the injectors

A lot of times is that the injectors are a little dirty, so you would need to diesel purge the system, and that cleans the injectors, you will feel the difference in performance as well as starting up.

I would also check the relay 109, I think Tdi parts, sells it between $10.00 to $15.00

Update:

I had someone adjust this, and I'm not sure if it's my imagination but the car is starting easier. I will hold off any glow plug replacement for now since I think it did the trick.

You could always look into a block heater. It involves heating the coolant slowly so that the engine is warm.

It never not started, it just took a lot longer. Seems to be better now.

Bringing this thread up from the dead, there is also an "afterglow" period. The glow plugs are still on for a while after the light goes out up until 2500 rpm. This gives cleaner emissions and is supposed to make starting easier. GP duration is controlled by the ECu, turning the key twice won't do anything.

Older cars had a preglow that was activated by the door, by the time the TDI came around, this was discontinued.