That is almost impossible to answer, as it depends on so many factors. The biggest one is probably how fast someone is coming out of high school, and secondarily, how long they have been training at a serious level. For example, an old teammate is a high school coach in Indianapolis. One of his swimmers (a sprinter) did not really start swimming competitively until his freshman year of high school. His times dropped by a phenomenal amount over 4 years, and I would think he could drop quite a bit more in college.
On the other hand, someone entering college that is already swimming at an elite level might not drop as much over 4 years of collegiate swimming. But even that will depend on numerous factors, including their technique (can it be improved significantly), whether they grow at all, how much weight training they've done previously (e.g., can they make significant strength gains), and (obviously) how hard are they willing to work. Some won't improve all that much, while others will make extraordinary time drops.
In other words, it depends.
As for 100 yard freestyle (NCAA is short-course, in a 25-yard pool), the NCAA record is :39.9 by Caleb Dressel. (That time was unfathomable when I swam in the 1980s). His :17.83 in the 50 (2018) is from another world. His second lap was :09.1. When he was a freshman (2015), he won the 50 in 18.67. He was only 11th in the 100 at :42.6 (I doubt that was his fastest time, though). Still that gives you some idea of the high end of time drop over a college career.
Back in 1985 the NCAA record was :19.3. I think there were only 1 or 2 in the Big Ten that went under :20, and only 5 or 6 in all of NCAA that did. In other words, Dressel could have started in the water and still toasted the entire field by over a body length.
Don't ask me to explain how times have dropped so much in 40 years - I have no good answers. Then again, my times from the mid-1980s would have won almost every event at the 1960 NCAAs (and even beaten Mark Spitz as late as 1971). By the time the mid-1980s rolled around, my times were yawn-inspiring.
Here's Dressel in 2018:
So, for guy