At 5 months old, they'll be small little nubs. At 8 months old they'll be a little bigger. By 10 months old they should be able to injure someone, and a little beyond a year, they should be an inch long or more. After 2 years and we're talking beautifully dangerous weapons.
Hens with spurs (yes, some hens can have spurs) will develop them to a maximum of an inch long by 1-2 years old. Sometimes you don't notice much development at all until then.
Some breeds take longer, too. Oriental Gamefowl don't get much of anything until a year old. Before then, they're just small little nubs.
edited to add - 9 months old is NOT a "halfway mark" unless you're getting rid of your chickens at 1-2 years old. The spurs of a rooster will continue to grow and grow well beyond 2 years old. Some are far over 3 inches long, some curl up and back towards the leg.