Does this use much battery - or is it something that can only be used for a short while before being switched off automatically when parked up? 

I had a TV tuner on my mk II that I often used when SWMBO went shopping.  It'd run for a while with the engine turned off, them it'd warn you that the battery needed charging, so I'd start the engine for a few minutes and turn the engine off again and continue to watch it either until SWMBO returned or the warning reappeared.

 

It's over 7 years since I had this car, so I can't remember how much time the TV would run for before the battery was depleted.

These only use a 0,5mm wire for the tv tuner so no more than 7 amps of current draw.

The radio itself will draw more power than the TV tuner.

 

Ive had the tv tuner on for an hour with the car off without issues.

One thing to remember, all of the vehicle control modules will be in awake mode, so current drain on battery will be considerably more than just the tv tuner.

6 hours ago, Warrior193 said:

One thing to remember, all of the vehicle control modules will be in awake mode, so current drain on battery will be considerably more than just the tv tuner.

 

With the ignition off?

Yes, that is what happens if a dashcam is left powered with the ignition off.