Hope this will help.....I used to promote guided tours of Delos, spend as much time as your appetite for architecture, culture, history, mythology can handle... There is a fantastic museum there now too!....go prepared, suntan lotion,hats lots of water, picnic also... plenty of time to take it all in.... Imagine what Delos would have been like all those years ago.....
Delos is an important island. You can hop on the boat and see around yourself (tour isn't necessary). I recommend to take an early boat (less warm, less crowded). We can easily spend 4 hours there, but it depends on what you want to see. If you just do the museum and the important houses (and the lions), you will spend an hour or 2, I guess.
Just check the timetable on the pier. Maybe you have to make reservations now due to Covid. I don't know..
I have never been but from what I understand, the best way is to either join an organized tour group or take the ferry from one of the nearby islands, like Santorini or Mykonos. It's rich with history!