How low should I go in terms of pad thickness?
Do I assume we all change disks every we change the pads time as these days?
What thickness are the pads likely to be when the warning light comes on?

When the light comes on you have a couple of mm left on the pad. I find he general rule of thumb is that you change he discs on every second pad change.  There is nothing that can't be fixed with a hammer😜😜
FFRR 4.4 SDV8 Autobiography Santorini Black.
Fiat 500x 1.4 multiair Lounge 2015
2010 LR D4 Commercial 2.7 TDV6

But if you change the pads before the light comes on then you'll save the expence of a new sensor.... 3 mm should be ok  Yesterday I couldn't spell Engineer... Today I are one!
Inventor of the 'Guide-o-Matic automatic wheel alignment tool'
Former long term L322 owner, Up/Down graded to a Classic Tractor!

Thank you for the replies, all good advice

Very true miggit  There is nothing that can't be fixed with a hammer😜😜
FFRR 4.4 SDV8 Autobiography Santorini Black.
Fiat 500x 1.4 multiair Lounge 2015
2010 LR D4 Commercial 2.7 TDV6