I recently went to a training with a friend of mine who teaches fourth grade (all subjects). It was the kind of training where somethings were useful but then there were large pockets of time where we were doodling, writing our "to do" list, and generally not paying attention. During that time I put together a rough plan of how I would spend the first few weeks in science and the rough timing for the rest of the school year. I'm posting it in case readers might find it helpful during the planning phase of summer (I am trying to develop an entire unit plan for my Teachers Pay Teachers site but I'm not sure I can get it done quickly enough).
The first weeks in science is dedicated to our inquiry standards. The general understanding is that our inquiry standards are introduced in those first few weeks and then many of the concepts are woven into our other standards (Weather, Organisms, etc.).
The problem with our inquiry standards is that they can easily eat up the first three weeks of school (particularly if you have under 40 minutes to teach science!). A lot of teachers do not want to spend that long in that unit and are often pushing to get to the first "real" unit of science. I would say two to three weeks is fair amount of time to introduce the basics of science after which all the other units would be anywhere from six to seven weeks long - which would take us up to our state testing dates and allow for a review period before our PASS test (each state is different - this is what would work for me in South Carolina).
This is my very unscientific formula I use for calculating my unit lengths:
180 days of school. Subtract 10 days for Inquiry/first days of school. Subtract 10 days for PASS review. We are left with 160 days of school. Subtract 20 days after PASS testing and you are left with 140 teachable days before our state testing. Divide that by 4 weeks in a month and you are left with 35 weeks in the school year. Divide that by 5 days of the week and you have roughly 7 weeks for each unit with some wiggle room.
Some teachers look at the amount of indicators they have with each standard and then adjust accordingly (i.e. some standards have more content then others). Whatever best works for you and your team you should do but I thought I would share how I calculate my general "lengths of unit."