I receive mail and small packages (not seeds) direct from Asia and Europe all the time.
While I'd say average shipping time from Europe or Asia is about two weeks, the time delay is HIGHLY variable. It depends on where they are being sent from, time of year, and for all I can tell, what appears to be dumb luck. In general letters run faster than packages. I've had small packages arrive from mainland China in literally 5 days, but I've also had packages take as long as 5-6 full weeks after confirmed shipping. I've had packages shipped from two different locations at the same time arrive weeks apart.
In terms of receiving seeds in the mail, generally its not worth Customs/HSA's time to chase these things too much.
You have to realize customs is screening for all kinds of things: actual drugs, explosives, dangerous chemicals, biologic agents, ammo and weapons, live vegetation (with infectious risk), foodstuffs, etc. They're already overworked and underpaid.
Yes, cannabis seeds are contraband, but ultimately they're just seeds, and by themselves pose no risk to anybody. Compared to the other truly dangerous stuff they're looking for, seeds are small potatoes.
Customs is a Federal agency, and while in theory they could turn a case over to the DEA, FBI, or even local law enforcement for investigation, there has to be a pretty good reason before they'd actually do it.
Those Federal agencies are also overworked/understaffed, and they have bigger fish to fry. The currency of "value" for DEA and FBI and the like are big high profile busts. They want to nail big drug kingpins and make big seizures or drugs, weapons and cash, that look good on the tube and get press. They don't so much care about Joe Schmoe growing a few pot plants in his basement. Unless they think its going to lead to a big bust, or they have some other specific reason to be interested in the party in question, these agencies aren't going to want to spend time chasing a few seeds.
Further, trying to actually prosecute a case like this could be tough. Just because seeds arrive at spot "X" doesn't mean the person who checks the mail at spot "X" ordered them, even if the package is addressed to them. Its tough to prove beyond a reasonable doubt that person "X" ordered the seeds, particularly if the one receiving the seeds denies it and there is no other evidence of growing.
If they want to prosecute you for possession, you have to actually possess the seeds. . .if the seeds have been seized, you don't possess them!
In theory, if they can prove that you ordered them, that could be construed as intent to manufacture/cultivate which is a crime. However, again, without other evidence of cultivation (which would require further investigation to develop, probably including issuing of a search warrant), a charge like that could also be hard to make stick.
Here's an example of what can happen when police jump to conclusions about this sort of thing. A bunch of cops assumed that package "X" full of actual marijuana (not seeds) was intended for the addressed recipient, and conducted a violent raid on his house, killing both his pet dogs in the process. Not only was recipient totally innocent, but he was actually the MAYOR of the town in question!
http://en.wikipedia.org/wiki/Berwyn_Heights,_Maryland_mayor's_residence_drug_raid
Anyway, usual customs policy on this is to seize the seeds, send a scary looking letter to notify the recipient that they've been confiscated, and let things go at that.
As for the US postal service, as mentioned, they aren't allowed to open first class mail without a search warrant, and unlike US Customs/HSA the post office's primary task is NOT to try and identify contraband, but rather just to make sure the domestic mail gets from point "A" to point "B" in a timely manner.
As a matter of practice, any postal branch will handle literally hundreds if not thousands of packages a week, and tens of thousands of letters. Something is going to have to look REALLY suspicious before a post-office employee is going to want to go to the trouble of actually opening up a formal investigation, and I don't think a small letter or package with seeds hidden inside would qualify. Also, in my experience, postal workers generally aren't looking for extra work to do or to stick their necks out.
