I'm in the middle of a major plot arc right now after three years of mindless filler, actually. It's very easy for me to measure, because filler for me is literally me doing nothing day by day. The first half of 2017, at least, has been a plot arc in my life.
But in terms of character development plot arcs, my entire life was filler up to December last year. Since that point it's been nothing but plot on that front. Which isn't a bad thing...at least, not yet, and not when I'm not...being me.
I don't like thinking of any of the suffering I have gone through in the past - or am presently going through - as necessary. I prefer to think of it as filler, because my outlook on life is already sufficiently bleak without thinking that I have to suffer to get anywhere. That's a little too much even for me.