Lisbon Airport Immigration delays?

So hard to predict. But you also have other potential issues -- and the big one is -- will the plane be on-time? I would schedule tour only if you do not have to prepay. If you have to prepay then I would not do it.

Are you flying direct from the US? If you stop in Paris or another EU country, you won't do passport control in Lisbon. If it's direct, then you will.
You write you want a tour of the outskirts of Lisbon. Exactly what are you wanting to tour? If you are doing a private tour, the company would check your flight status before coming to the airport, so a delay wouldn't be such a big deal. And, if your check in was delayed a bit, it shouldn't be a problem.

I also have read some things on delays at the airport, but, I've also read reports of getting out of the airport upon arrival in around 40 minutes.
If your time here is limited and you work with the tour company about what happens in the case of delays, it should be doable.

All that said, if you are referring to Sintra, a half day tour wouldn't be enough anyway.

I landed in Lisbon coming from the US at the end of May. The lines were extremely long. I would guess around 1,000 people were in line. I know that number seems outrageous, but I was so bored standing in line I counted how many people were in one lane and then counted all the lanes. It was definitely over 900. I think I was in the line for about 45 minutes. They had a lot of agents at around 10:00AM and the line moved. It just depends on how many planes land at once and how many agents are there. It was definitely the longest immigration line I've ever seen entering Europe.

I went through Lisbon airport last summer in July. It was a madhouse. The lines were really long. And they didn't seem to be moving very fast. The airport seems to be having a hard time handling all of the new tourists rushing to the region. Luckily, for us, we had twin babies and a stroller, so we were rushed to the family immigration line and didn't wait at all.

Thanks for the input everyone. The article I read said it was 2 hours plus to clear the 1000+ backlog. I arrive at 10:30am so I am hoping I will be ok to get out of there within 2 hours.

I'm going to try to get a private tour to Sintra (want to see Pena Palace and Quito De Regaleira) with a pick up at 12:30 at the airport and drop off at my apartment in Chiado at 17:00. Is this recommended?

Bear in mind that Sintra will be extremely crowded - especially since you are going in the afternoon. Unfortunately, all those people that clog up the airport visit Sintra, too!

I'm also not certain you can see both Pena Palace and the Quinta in that amount of time. My first trip to Sintra was a small group tour. We saw Pena Palace and the Quinta but it was a day long tour. A private tour could be different, though. I encourage you to ask questions of the tour company regarding how long you visit both places, etc.

I traveled to Lisbon in mid-June (direct flight from Boston). The lines at immigration/passport control were LONG. We landed around 6:30 am and I didn't get to my hotel until around 8:45 am.