I usually allow 20 days for items to arrive from the USA.
There can be variations if there are an unusual number of weekends or holidays in that period. For example, an item purchased on Friday August 29 by September 21 would have passed through three weekend and Labour Day slowdowns.
And if you don't live in a major centre like Toronto or Vancouver, it could be longer.
You have up to 45 days to start an Item Not Received Dispute and 60 days to leave feedback if the problem is the seller delaying shipment.
Also, are you buying from new sellers, or buying in high value/high risk categories? Some sellers who are subject to the Paypal 21 Day Hold will decide they don't have to ship until the money is in their own hands (although the cleared payment is with PP). This is a common delay.