case with date diff

Edit: as @shawnt00 mentioned in a comment on the question, the problem is most likely the fact that you're dividing integers (resulting in an integer too, ie 0 or 1 in this case). Adding a * 1.0 should fix the problem (or a cast to float8 if you prefer).

You can use the FILTER clause for this:

select count(*) filter (where date_diff('day', min_time, max_time) <= 7) * 1.0 / count(*)
from table 

Though since it looks like you're computing an average maybe something like this would be the easiest:

select avg(case when date_diff('day', min_time, max_time) <= 7 then 1.0 else 0.0 end)
from table