How much can break pads wear before they start squeeking?

I'm guessing until they wear down where the notches across them disappear right?

If you're talking about the "change me now squeeler" that comes mounted to one pad in each set, that all depends on the pad (i.e. where the metal arm start touching the rotor)... but you can probably get a month or tow of daily-driving out of it at that point, don't expect repeated braking though since it'll cook pretty quick when the pad is that thin.

If your talking about general dust/pad squeel that's pretty much determined by pad material. You can help things out by placing a small dap of brake caliper silicone on the back of the pad's backing plate, and ensuring all your anti-rattle springs are tight and in place. A good water-rinse and a couple hard stops can help as well. Some pads squeel like a freight train, others are as quiet as library.