I did some research into when diminishing marginal returns to exercise kick in. I eventually found this excellent study http://ije.oxfordjournals.org/content/40

I did some research into when diminishing marginal returns to exercise kick in. I eventually found this excellent study http://ije.oxfordjournals.org/content/40/5/1382.long http://ije.oxfordjournals.org/content/40/5/1382.long which indicated to me that ~1000Cal/week expended in a vigorous fashion should be what I shoot for. This means that stuff that elevates post exercise oxygen consumption like resistance training and interval training for cardio are great bangs for your buck. This jives well with other research on the health benefits of these activities. I'm currently hitting the threshold with 2 days/wk of HIIT and resistance training each.