For international flights out of Seattle Airport, the major airlines suggest getting to the airport 3 hours before your flight. For domestic flights, 2 hours should suffice. It's work knowing that as one of the busiest airports on the west coast it can have amongst the longest security lines, though the airport scores well for flight cancellations and delays.
View the SeaTac TSA Wait Times and PreCheck lane information above to see which checkpoints are open.
Not already enrolled? Reduce your SeaTac wait times and find out more here https://www.tsa.gov/precheck.
The Port Of Seattle website itself states that SeaTac security lines for non US citizens arriving at Seattle are typically an hour or longer. You can expect very long lines at SeaTac during the holidays and other busy times.
As with any busy airport a key factor you should know in how long the security lines are at SeaTac for arriving passengers is time of day and how many other flights are arriving at about the same time.