That is a bit like asking how long a piece of string is. It is not really an answerable question. For a beginner even simple words can be difficult. As your vocabulary improves, so words become easier.
I think difficult words are those with 4-5 syllables for they are not so commonly used in conversations. And also those words that are rarely used but in deep literatures and writings...
I think it's hard to predct what people will need, but there are some lists. If you look here, you will find some lists that people have made, including an academic list taking words commonly used in academic writing, etc: http://www.usingenglish.com/resources/wordcheck/
Difficult Words! 
Linear Mode
