Well in depth guide will take a while to record/write, but generally yes - when hauler spawns (t1 or t2, its called impel, badger, mammoth and so on) it means that miners are approaching full cargo. its 5k for ventures 10k for retri and 10k for skiffs.
First hauler spawns after 90mins of fleet spawn and then are locked in 30 mins intervals (even if the ships are 100% full before that timer hauler wont spawn).
And here is a clip i recorded couple years ago, just testing things out and its not a guide or anything like that - just goofing around. At 1:10 you can see what dropped from the hauler, i killed him before i started the recording.
Right now i oprimized the fit and approach so its taking no more than 5 mins to kill ventures/retri and about 16mins to kill skiffs (they are brick tanked).
I think ill record a guide and talk about those fleets, cuz alot of people dont know this but miners also drop storyline meta modules worth 25mil, 100mil and 400mil. Laser upgrades. Even from ventures.
But i am afraid of ccp nerfing it, its so profitable that shouldnt be allowed in HS.
And another interesting clip, check out how much ore t2 hauler has 
EDIT: i know, clip is blocked, i removed the music from it and waiting for youtube encoding. 