Brakes Disc brake pad wear



I have to demonstrate my ignorance here. About how long do the brake pads for disc brakes last? Do they make some noise or something to let you know that you're done adjusting and need to get to replacing? I have a little over 1,000 miles on the brakes on the trike. I've adjusted them several times. Will I have to disassemble them to determine how much life is left in them?
I'm thinking automotive here for I never owned a bicycle disc brake. Isn't there some kind of window that you can see how much pad you have left?
I'm thinking you're gonna have to just take it apart once and note if it has a warning strip like in cars that will make your brakes squeal when it's getting low or if you can find a spot where you can see how much you got left.
Okay, I guess the next time I have a few spare minutes, I'll disassemble one and see what I can see. I'll let you know if I see anything of interest.
Just a thought...measure a new pad before you pull your old ones--this will help you determine wear and life. If new pads are 1/8" inch and your pads are 1/16"--you won't be shocked--and you'll know that you should have another 600 miles or so left.
I still haven't looked at my pad wear, but another idea occurred to me. Has anyone set up hydraulic discs as a single pull system? I've never owned a set. Just thinking at this point.
if pad gets worn down that much . you will hear the noise metal too metal noise. why dont you just take one of the pads out an have a look