Why does my bike take so long to reach normal running temperature?

The ninja runs cool; the temperature needle will normally stay very close to the cool end of the normal range.

After startup, don't wait for the bike to warm up at idle; this will take too long, waste fuel, and encourage carbon buildup on the valves and spark plugs. Instead, start it up, give the engine a few moments to get into the rhythm, then ride away. Do not apply excessive throttle, or make sudden throttle changes; be smooth. Keep the tach below 6k until the needle is over the cold mark, and then below 9k until it has reached the normal range (6k and 9k are arbitrary limits; you must choose what works best for you). Riding the bike will help it warm up much more quickly.

Additionally, during the colder months, a piece of cardboard can be placed over the front of the radiator to partially cover it; this will make the radiator less efficient, and increase the rate of heat build up. If after doing this the temperature needle goes over the mid-point while riding, use a smaller piece of cardboard.