There isn't a pre built tool but what you do to get a decent estimation is you look for what engine/language the VN is built in and use a macro to count all instances of in game text. All engines have parts in the code which tell it this is a line of text, this is a line of text, you just use that to count them in moments.
Such as for this game it's using Nscripter, so all lines of text are usually going to end in a "\" Now the \'s also are used for defining file locations so you have to have it only count \'s which have punctuation before them. so "?\" "!\" "。\" etcetera, (this can be thrown off a bit if the VN doesn't end it's lines with punctuation, but from what nearly all Japanese VN's end lines with punctuation.