I do not know why people keep posting 2-4 weeks?
30 days is the minimum quarantine for most nasty's based on a 21 day life cycle.
We assume that in that period people look at their fish and start treating if they see any symptoms. What you say doesn't make sense; if most such critters have a 21 day life cycle, why on earth would you need 45 days quarantine in order for symptoms to show? Unless you put them in the quarantine tank and ignore them, you're going to notice something before then.
Obviously if the fish is sick you're going to quarantine longer. 2-4 weeks gives you long enough to see if the fish is sick.
EDIT: I think that we're talking about slightly different things. I assume now that you mean that if the fish is sick you should quarantine minimum 30 days. But what other people are saying is that 2-4 weeks and NO symptoms generally means that the fish is okay. Quarantine for verified sick fish vs quarantine for new and possibly sick fish.