DateDiff with Conditions

Well, you have two problems here. First, you need to flag the correct 500 rows to do your calculation on. Second, you need to do your calculation.

The first part is something like this:

IsFirst500After200 = 
    VAR __Table500 = FILTER(ALL('Table'),[Response code] = 500 && 'Table'[Index]<EARLIER('Table'[Index]))
    VAR __Table200 = FILTER(ALL('Table'),[Response code] = 200 && 'Table'[Index]<EARLIER('Table'[Index]))
    VAR __Max500 = MAXX(__Table500,[Index])
    VAR __Max200 = MAXX(__Table200,[Index])    
    VAR __Flag = 
        SWITCH(TRUE(),
            [Response code] = 200,FALSE(),
            __Max500 > __Max200,FALSE(),
            __Max500 < 'Table'[Index] && ISBLANK(__Max200),FALSE(),
            TRUE()
        )
RETURN
    __Flag

The second part:

Duration = 
    IF(
        [IsFirst500After200],
        VAR __Next200 = MINX(FILTER(ALL('Table'),'Table'[Index] > EARLIER('Table'[Index]) && 'Table'[Response code] = 200),[Event time])
        RETURN DATEDIFF([Event time],__Next200,SECOND),
        BLANK()
    )

This second part is the identical technique used in MTBF. http://community.powerbi.com/t5/Community-Blog/Mean-Time-Between-Failure-MTBF-and-Power-BI/ba-p/339586. 

I have attached the PBIX file.