I've found a useful technique is to switch between the original and the edited version. By doing this even minor changes sometimes look drastic, which could work against you sometimes, but it's a perfect representation of just "how far" you've gone with the edits. You can see how true you're staying to the original photograph and how unnatural things start to look even if that is your intention.

Taking time away from the photo as you mentioned is a really great method too, but as you said their isn't always time for it. With the same idea in mind, if you have other photos to edit you should move on to them once you're a point you're happy with on the current picture. Once you've gone through editing all the photos, start at the first and work your way through again. That should give a little reset time on each photo.

What exactly "too far" is, is up to you as the artist. But I mainly use the first technique as a gauge to see just how much editing I've done.