I usually connect my Android phone to my laptop with USB and leave it even after fully charged. Is it OK or harmful ? Will it reduce my battery lifetime ?
When your phone is completely charged, it switches to a trickle effect. This means that it'll draw only as much power from its power adapter as it needs to keep its full charge. Your phone still expends energy as long as it's on, even if it's plugged in. Whenever the battery is fully charged, however, it simply stops charging and will suffer no ill effects. Your battery will probably degrade from the effects of time before it experiences any issues from charging overnight.
So in turn, you are fine to keep it plugged in, even after it's charged, I always unplug mine personally just to be safe, but that's just me.
Keeping it plugged in is always better as the battery life depreciates more rapidly when you use the phone unplugged. Also, never let you battery run out or get low. It is always better to charge it whenever possible in order to maintain a longer healthier charge and life. This is opposite of what we have been told in the past in older batteries for example in laptops during the turn of the century where it was better to let you battery fully drain as to keep the timing mechanism in check however again this is not the case in the newer lithium batteries and it is always better to charge whenever and whenever you can. go here for more info: http://batteryuniversity.com/learn/article/how_to_prolong_lithium_based_batteries