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. More info
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 6000 until the needle is over the cold mark, and then below 9000 until it has reached the normal range. 6000 and 9000 rpm 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 buildup. If after doing this the temperature needle goes over the mid-point while riding, use a smaller piece of cardboard.