Can someone give me some information about how hard is it,
It is not difficult, as long as you know all basic UML diagrams, like class/sequence/state/deployment/component diagrams.
Most of the questions are scenerio-based, and thus, you have to read some cases, and choose the most suitable diagrams that represent the case.
and how long it take to prepare for it
I learnt UML in my undergraduate days, and I had 3 years OOAD experiences. I used 1 week to read through Fowler's UML distilled 2nd edition to refresh my UML knowledge, and Larman's Applying UML and Patterns to refresh my OOAD knowledge.
Hope this help.
Nick
SCJP 1.2, OCP 9i DBA, SCWCD 1.3, SCJP 1.4 (SAI), SCJD 1.4, SCWCD 1.4 (Beta), ICED (IBM 287, IBM 484, IBM 486), SCMAD 1.0 (Beta), SCBCD 1.3, ICSD (IBM 288), ICDBA (IBM 700, IBM 701), SCDJWS, ICSD (IBM 348), OCP 10g DBA (Beta), SCJP 5.0 (Beta), SCJA 1.0 (Beta), MCP(70-270), SCBCD 5.0 (Beta), SCJP 6.0, SCEA for JEE5 (in progress)