Consider the unfortunate French abbreviation, S.D.F., which normally means "sans domicile fixe". Those three letters, for our purposes, represent since, during and for. Here's how that's going to help us:
[I haven't seen him since last week]... from a starting point in the past until now. A starting point can be an hour (6am), a day (Thursday), a year (1981), etc. Another trick: when you write since, you place a dot, a point, above the letter 'i'--so we use since with points, with starting points.
Example: I haven't been to the movies since June. (June is the starting point)
[My boss slept during the meeting]... we define how long a meeting, lunch, holidays, etc, are. We can have a 5-minute meeting or a 5-hour one. A 10-minute lunch or a sumptous, 2-hour lunch. These durations aren't fixed, they can be longer or shorter. And they usually have a name like meeting, lesson, summer, holidays, match, etc. Another trick: if in French you can say "au cours de," then you can use during.
Example: We decided to have lunch during the football match. (We can say 'au cours du match de foot')
[We have lived here for 6 years]... six years is 312 weeks, or 2190 days. It's a fixed period. 2 minutes is fixed as well. You can't make it longer or shorter unless you go into space. Another trick: when you write 'for' you cross the letter 'f', you cross it with a perpendicular line, that line represents a duration (as opposed to a point). For is used with a duration (never a point), a fixed duration.
Example: They decided to stop and stretch their legs after driving for three hours. (Three hours can't be stretched or shortened, three hours is a fixed period)
English, like any other language, isn't a hard science and some of it doesn't follow the rules as well as we'd like it to. But most of it does, and I hope that these little tricks will help you use SDF correctly more often than not.