When you'll get it. Unfortunately, there's no hard and fast rule. Different people are susceptible at different elevation and one person might react differently on two separate occasions. Lots of factors enter into the equation, and scientists don't fully understand them. It's common, however, for fit hikers to feel the effects of altitude at about 10,000 feet.
Some people feel the effect of elevation much lower.
Age, fitness, pre-existing medical conditions, and medications may play a role but the person who is most at risk is the person who climbs too high too fast. Being super-fit doesn't exempt you. In fact, it may increase your chances of AMS because you're capable of climbing too fast for your own good. Try to limit your net gain of elevation. There are no hard and fast rules.