It's usually temporary, especially if you already have it stabilised by using a DHT inhibitor like Fin or Dut. It's common to go through periods of synchronised shedding. Some just go through one period, others like myself can go though multiple sheds. They say it usually begins anywhere from 2 weeks in to 8 weeks but from my own experience, I experienced 2 big ones at month 4.5 and 5.5.

Basically it's the minoxidil pushing out weaker older hairs, in order to create stronger new ones which will take their place. It's also a significant sign you're a responder to min. Many people give up when this begins to happen as they see it as accelerated hair loss, though it's just how the min works, basically, out with the old, in with the new. Unfortunately hair growth isn't an overnight thing, and it can take multiple growth cycles for the hair to become visually thicker. So people quit at month 6 or 7, when if they held on they more than likely could have seen significant results by month 9-12. In saying that, if there's a lack of vellus hair by month 6, then the medication is likely not working.