You know enough is enough when: 1. you feel shaky and want to get off the bike (or road, out of pool) and leave. There's a variety of factors which can add up to equal days like this. There's no shame in saying "whew, ain't got it today," and stepping out. In fact, that would be the prudent, intelligent, and sensible thing to do (but not the easiest thing, of course). It's the old triathlete problem: If I miss one workout, my entire schedule is wrecked! Not so. Adjust for how you are feeling. Don't feel like getting out of bed? Sleep. Don't feel like doing your interval workout tonight? Go easy with strides. Obviously, this kind of 'do it when I feel like it' attitude only works for people that are very honest with themselves and very strict about their training. Ie, Joe Blow from Idaho would never do an interval session because 'he doesn't feel like it.' You could take an honest assessment of the day/week, and say "I can do this tomorrow as opposed to today with no real trouble." All that being said, the flip side is that their is something to be said for pushing through times when you feel like crud, as you're not always going to feel good in the race (hopefully you would, but unlikely). Toughing it out is good mental training. It's up to you to figure when it's 'mental training' and when it's 'being a compulsive-obsessive triathlete.' Seinfeld: "Not that there's anything wrong with that!" Marty