for quite some time now, I am wondering as to the extent one must use the {qaStaHvIS}.

my understanding is, that if we are talking about an event which takes place once, there is no need for {qaStaHvIS}.

example:

DaSjaj matlheD

(on) monday we depart

on the other hand, if we talk of an event that lasts quite some time, then the qaStaHvIS is (seemingly) required.

example:

qaStaHvIS DaSjaj jIvum

while monday is happening I work

so, here is my first question: in the aforementioned example {qaStaHvIS DaSjaj jIvum}, is the {qaStaHvIS} necessary ? can't I just write {DaSjaj jIvum} ?

Yes, you can just write DaSjaj jIvum I work on Monday. You're not telling us about how your work was structured, just that on Monday, work happened.

maj..

moving on, my next question is "if we are talking of singular events which take place multiple times, do we use the {qaStaHvIS} ?"

example:

on monday I kill four birds

do I write 

{DaSjaj loS bo'Degh vIHoH}

or do I write

{qaStaHvIS DaSjaj loS bo'Degh vIHoH}

For simple time expressions, this doesn't make a difference. qaStaHvIS is much more useful when referring to events, e.g. qaStaHvIS SoQ jIQongchoH I fell asleep during the speech, or time periods, e.g., qaStaHvIS wej DIS toQDuj baHwI' jIH I was a Bird of Prey gunner for three years.

Exactly when a phrase is a time expression or a time period is difficult for me to articulate. Longer periods get treated like time periods when the subject is events of short duration (like a single fight during a period of three years). qaStaHvIS has a sort of "zooming-in" effect; you are examining a subset of the entire period and saying that the period was going on before the point you're looking at and continues on after the point you're looking at. When you want to zoom in, treat it like a time period and use qaStaHvIS. When you just want to label when something happened, and you don't want to zoom into the context of the moment, use a time expression.

-- 
SuStel
http://trimboli.name