Usually, in most cases, my experience is that it only stays on that steps for a short time such as a few days to one week.
However, it may vary greatly with no consistency or predictablity in system; i have seen some only be there for less than one day but others take multiple weeks (such as the exmaple of my own pack I showed previously that stayed for a month in customs then updated and moved without any issues -- pic below).
That was a personal example of one of my own that i saved a screenshot but I have seen others examples that took even longer (I think someone recently mentioned successfully receiving one that was held for multiple months) and still made it without any sign of being opened or any problems -- so you still have a good chance it arrives.
Until you receive a letter from customs/homeland-security (proff it was seized) there is always a chance it is simply delayed and will still arrive.
With that said, in general it seems that the ones that take over a week or two in customs usually have a higher seizure rate. I feel that if it sits for over another week, while it may still move later, there is a realistic chance it has been seized.
In addition, some points of entry are worse than others. From what I have seen, NY and Miami seem to be lower seizure rate, while Chicago seems to be the worst, for instance; take the point of entry into consideration if trying to predict the likleyhood of whether or not it was sezied.
I am waiting on a couple new packs myself and they seem to be delayed a bit (not delayed from the source, I mean the postal system has them); hopefully we all get update and they move soon.
Remember, Basicstero will give a one free re-ship with proof of seizure so while the delay is inconvenient, the source eats the cost for another try and at least you may not be out the value if the re-ship is a success.