How long to clear customs in Porto?

Welcome to the forum! The thing that takes longer is Immigration (which is for people). Customs is for goods. Are you arriving at another European airport first or is your flight directly to Porto? Asking to see if you will go thru Immigration at another stop which is where time might be spent.

For actual Customs (not Immigration) you just walk thru a green door that is nothing to declare so it takes no time at all.

Thanks for the info and clarification - guess the question should be, how long should I plan for it to take to clear immigration at the airport in Porto when coming from the US?

Are you arriving on the UAL flight at 340PM or the TAP flight at 905PM? ( those are the only US-Porto direct flights I could find)

I suspect the immigration lines mid afternoon would be longer

We are meeting friends that are arriving on United Flight UA144 that arrives at 9:45am.
Trying to determine an approximate time they will need to clear immigration.