How long should I spend each day on learning new words?
Obviously, people vary and the way each person likes to study may be different, but good advice is to study vocabulary in short, regular sessions. Thirty to forty-five minutes at a time is probably sufficient, otherwise you may lose concentration. It is important to be fresh, so that your brain retains the words you are studying. It is no good spending hours studying only to find that you have forgotten much of what you studied!
If you are really short of time, then it is better to break your studying into several short periods than to have one long session. Remembering is an important part of learning vocabulary and you will remember better if you have short study sessions and review what you have learnt frequently. Carry a list of the words you are studying in your pocket and pull them out from time to time during the day to review them.