Mine will clear a misted up screen on the inside in about 30-60 seconds.
To defrost a frozen outside, takes around 2-4 mins depending on how thick it is.

I notice when the FBH kicks in that the screen thaws quite quickly though from the bottom up. ON THE WAY: BMW F21 M140i
MERCEDES GL350 - Metallic Navy Blue w/Grey interior

GOING: BMW F31 335D MSport+ xDrive Touring
GONE: Land Rover Discovery 4 HSE - Stornaway Grey w/Black Interior (wish we'd kept it )
GONE: BMW F10 520D MSport - Carbon Black
GONE:MY12 RRS SDV6 HSE Sumatra/Ivory/Grand Black Privacy glass

Ta  RRS 2010
3.0 TDV6
HSE
Auto
Santorini
Ivory Leather

I too had the line in the centre and thought I had a fault - until I read about the two circuits on this forum, then looked very closely to see there is a gap between the elements!!