Few of us are lucky enough to do all systems engineering, all the time. We frequently divide our time with clerical tasks (e.g., typing someone else’s requirements into DOORS), management tasks (writing personnel reports), and business development. There are aspects of these that may fit under SE, but they frequently are simply not relevant. We rely on your judgment to draw the line. As a guideline, we do not expect someone who manages a team of 25 people to be doing SE full-time. Similarly, it is rare (but not unheard-of) for someone to be doing full-time, professional-level SE at the beginning of their career with no degree and no prior work experience. In both these cases, we would expect you to pro-rate your experience.
Let’s say you had a period of 2 years where you spent half your time in a non-SE role and half of your time doing SE work on a special project. In this case, you would claim 12 months (24 months * 50% = 12 months) of time in SE. You’ll then break that down further to allocate your SE time across the work areas described in the application instructions. E.g., 8 months in Requirements Engineering and 4 months in Process Definition. You should describe your math in the Duties and Responsibilities section of your application alongside the description of the work you were doing. It commonly takes 7, 10, or more years of work to achieve 5 years of SE. You will need to have references to cover the entire minimum period of experience.
Does the reverse work, that you can count extra if you worked more than a 40-hour work-week? No, I’m sorry, it doesn’t. The most you can claim for any one month worked (regardless of the number of hours) is one month of SE.