last i heard google will crawl and index urls well over 1000 characters. http://productforums.google.com/forum/#!category-topic/webmasters/crawling-indexing--ranking/h0aeO2_kpac
... but that doesn't mean that it's a good practice :-).
i would limit it to a url length that is readable in a search result which would probably give you better click-through. make all the filenames lower case. lose the underscores and use hyphens for word separators.