We don't have an end date either. We school year round, work on something until it is done and then move to the next level or program.
I wouldn't want to deal with the stress (on me or my son) of trying to cram something in by a certain date. I do have an idea in my head for when I want to start certain things (like Latin) but don't worry about getting a certain piece of our curriculum done by day X. We also don't use terms like 1st grader, 2nd grader etc. There is really no moving up from one grade to another.
We take breaks when we want/need (emergency appendectomy anyone?) and usually take an extra day or two off a week during the summer. That is in addition to the fairly regular traveling we do. We get plenty of schooling in throughout the year and it all works out in the end (whenever and wherever that might be).