This is an interesting question, but before we can explore the answer we must ask ourselves what is intelligent life.
Earth creatures with nervous systems and language include whales and gorillas. Whales are found to have languages with more words than the average human uses. Their language, culture, and habits vary from one part of the world ocean to another. Gorillas not only have their own language, both verbal and body language, but can learn sign language to communicate complex thoughts and emotions to humans. Octopuses are said to be a very intelligent species. What about my dog? If a planet had dogs, would it mean it had intelligent life? If a planet had whales or gorillas, would we call that intelligent life? Does intelligent life only mean a civilization that can launch objects into space or does it include Genghis Khan who was born in the year 1158 and ruthlessly conquered much of the world?
Earth is almost 5 billion years old, but almost all of that time has been without humans living on the planet. Archeologists have discovered 300,000 year old fossils of Homo sapiens and the first stone tools, which is only a third of a million years ago, what to speak of billions of years. Out of those hundreds of thousands of years when humans lived on Earth, it wasn’t until very recently that we started recording history and writing things down. Before that time, would you call them an intelligent species?
Suppose you visited Earth 250,000 years ago and saw homo sapiens living in caves and hunting with spears. Would you call them an intelligent species?
It is possible that many advanced civilizations in the universe have come and gone or have yet to develop. Some of them may be more intelligent than humans, some less. Some may be extinct by now, some have yet to evolve. Some of them may have sent radio waves out into space 20,000 years ago, but because they lived 150,000 light years away on the other side of the Galaxy, it will still be another 130,000 years before those signals reach Earth. Will any humans be around to hear the signal? Some aliens may have sent radio waves out a billion years ago, and those signals zoomed past Earth at the speed of light long, long before there were any humans here, and now that we’re here those aliens are extinct. Our own radio waves coming from Earth have only traveled 120 light years into a galaxy that is 150 to 200 thousand light years across. Will anyone ever hear them?
The question was: “How intelligent do scientists expect intelligent alien life forms to be?”
We don’t even know yet if single celled microbes have developed on any planet in the universe other than Earth, but we’re searching. That’s one of the missions of the new Perseverance Rover that NASA just landed on Mars: to search for past evidence that single celled organisms might have come and gone from there. If we find that then we will probably suppose life has developed on other planets, and maybe some of those planets might have some kind of organisms like plants or animals or even creatures that might be intelligent to one degree or another. Maybe they are just throwing spears to hunt or maybe they are sending probes into space. Maybe they live at peace or maybe they have annihilated themselves by nuclear war or polluted their planet to the point it’s no longer able to sustain life. Maybe they lived a billion years ago or maybe they do not yet live.
We don’t know what to expect.