How many weeks of instruction do you plan for getting through
All of it.
There are unplanned interruptions all year long. Some years the students are more ready for some content and go faster through it. Some years students need more time on a unit. Sometimes I'm sick for a couple of days. I will map out each quarter and aim to finish the last graded assignment 3 or 4 days early and start content that will be assessed during the next quarter. This gives me a buffer of a couple of days if something takes longer than planned. If I'm not hitting certain targets during the quarter, I have to make choices about which activities to modify or skip. The students still get access to all content, but maybe I switch some activities from inquiry to direct instruction. Maybe I skip a 10 minute video. Maybe I give the 20 minute worksheet as homework instead of classwork. If I'm ahead of pace I will add some extension material or give more class time on a project. Testing happens every year. It's not hard to plan around it.