As Mike says above, it's almost impossible to guess how long a particular set of pads is going to last, just too many variables. I've tried to be "helpful" in the past when asked by a valued regular customer and then subsequently catching a big cold when they turn up on the forecourt with a "horrid grinding noise". It's worth also considering that many mechanics will recommend a pad change some time before it's strictly due in order to avoid exactly that problem. Also, with long service intervals now a days, many will recommend a pad change even though plenty of material is still present because they feel they genuinely can't be sure the pads will last another year, 18 months or two years or whatever until the car is next in the workshop.
People such as us can do an inspection whenever we like so allowing us to get a lot more life out of our friction materials. I'd caution against trying to run things too close to the bone though, personally I'd not go much below 3mm and I wouldn't be setting off on one of our trips to "the deep south" unless my pads had considerably more than that on them.
Thickness of new pads does vary a bit - I'm not really in the habit of measuring new pads so can't be specific and I wouldn't be surprised to find they vary a bit from one manufacturer's vehicle to another - ie. thickness on a Jag or Range Rover might be different to a Mini or Seat Ibiza? However I'd take a guess (Mk 1 eyeball) that, on common every day stuff, it's going to be around 1/2" (13mm) give or take. One I did recently was my older boy's Punto (see pictures above near the beginning of this trhread). I serviced it towards the end of last summer - when it was still nice and warm weather wise - and noticed the pads were low enough for me to say to him that we ought to be doing them "soon". The Punto is his daily work car doing a half hour there and half hour back journey into the city roughly 5 days a week (he does shifts) with one very steep hill of approx a mile in length which has traffic lights at the bottom so needs a hard stop at the bottom of the hill so not all that friendly for the brakes. He eventually let me have the car to do the brakes about a month ago and I doubt if it had taken more than around a mil off the pads in that time. I just measured them (they are still in my box of stuff to go to the recycling centre) and they've got about 4mm still on them and, although they would have done another month or so, I'm very happy we changed them when we did. Why? because you have to be aware that when the friction material gets very thin it doesn't deal with the heat so well and will fade more quickly under arduous conditions. It's also more likely when the friction material is thin and especially if the pads are quite old - I suppose maybe over 6 or 7 years since fitting, but difficult to be specific, let's just say "old" as in "not in the first flush of their youth" that the friction material may separate from the metal backing and give you instant metal to metal contact, which will quickly ruin the discs to say nothing of reducing braking efficiency!
So if a new pad has around 13mm (1/2") of friction material and yours were reported with 6mm still on them it's tempting to think of them as half worn? But no! If we take it that they need changed when they get to around 3mm then at 6mm they are 2/3 done, yes? so still got quite a bit of safe running in them - my boy would have gone through to next summer if his had measured 6mm at the end of last summer - so no need to "panic" immediately but a check after a month or so and compare how much has degraded in that time against how much is left will give you an indication of how long you can afford to leave them. Oh, and remember to check the pad depth on all 4 pads because wear can be uneven due to differing disc surface smoothness and stuff like siezed/partly seized caliper sliders and pistons etc. It's not all that unusual to find inner pads wearing quicker than outers so don't just look through the weel spokes at the outer pads on each side and assume the inners will be worn the same.
When I was younger it was much more of a thing to regularly check pads because if you caught it early enough all you would need would be a set of pads and it was not unusual for you to get two or even three sets of pads worn out before a disc change was needed. Today it's not all that unusual to change discs and pads as a set every time and certainly you'll be very lucky to get more than two sets of pads worn out before renewing a disc.