How much time require to develope XEP-0198 ?
I'd say if you want good quality code with xep198 clustering support, then it would take a reasonable skilled developer with certain knowledge of xep198 and openfire at least a month to implement it and likely another month of bug hunting and fixing, ideally together with a community review of the code.
But if you are into software development, you know that such estimates are comperable to blindly throwing darts on possible answers to this question.
Othere than openfire developer can try to develope this ?
Everyone is free to try and make it's own experiments. 
If i dont know any thing about openfire source code than can i understand openfire project and try to develope XEP-0198 ??
Of course you can try, if it succeeds depends on your skill level and how much time you are willing to spent at it.
Can we integrate XEP-0198 through plugin ?
Unlikely with the current way plugins work, but may not impossible. OTOH experience shows that xep198 code works best as part of core functionality.