I would estimate it takes most of all time. Mailorder is something that cumulates over the years and returning customers - if they feel worth returning. Easy to fuck up that. In times like this, when it seems hard to estimate what will be state of underground in next.. 5 years? Gamble of how much and what to "invest" when usually it does take years and a lot of losses before all the deadstock problems may (or may not) start to resolve itself.
As for promotion, I would guess it is the only actual challenge, that is very hard to solve. No matter what size company or band you'd ask, everybody is struggling with challenge how to get word out there and how to be visible.