Holli,
Timing and planning curricular segments is both an art and a science. We begin by estimating the expected time for various topics, but as you suggest, sometimes more (or less) time is needed. I rely on "need to know" and "nice to know" activities. In doing so, I nearly always have enough time for what students need to know, but if additional time available, I can go the supplementary activities that are useful and regarded as only filler time.
Barry Westling
