Time period in Past Continuous is the same as point in time, but this point is just stretched into a period.
|Period in time||Point in time|
|He was watching movies all night.||He was watching a movie at 5 pm.|
The 3 conditions when we can use Past Continuous with time period:
1. Action was taking place in the past
2. Action was incomplete (i.e. it was continuous)
3. Indication of time period during which the action was taking place.
Words to indicate time period in Past Continuous:
all night/day long
from 5 to 6
during his studies
after 5/school/WW2 (end point is missing)
before 5/school (start point is missing)
till (late/early) morning/evening, till 5 pm (start point is missing)
until (start point is missing)
I was watching a movie until midnight
prior to (start point is missing)
When end or start point is missing
The time period can usually be inferred from the context, or the time period is just not important. The important part is that we know about the fact that the action was ongoing for some time in the past. This fact alone gives us the right to use the Past Continuous, even if the time period is not specified.
Also on Past Continuous: