if I want to say "for many years, I have my phone's sound level set at maximum", then there are two options:

{DIS law', ghoghwIj HablI'vaD wab 'aqroS patlh vIlo'}

or

{qaStaHvIS DIS law', ghoghwIj HablI'vaD wab 'aqroS patlh vIlo'}

if I understand correctly what was said so far, then at this example the {qaStaHvIS} is unnecessary, right ? we can use it if we want, but it isn't necessary, right ?

No. In this case you need the qaStaHvIS, because DIS law' is only a time period, not a time stamp. It tells you a duration; it does not pinpoint a moment.

You could say ben law' ghogh HablI'wIj 'aqroS wab patlh vIlo' but that would mean many years ago I used my phone's maximum sound level, but that doesn't tell us anything about how long you did this.

-- 
SuStel
http://trimboli.name