In vanilla game they will occur pretty often (for me it was like twice a week or once a week), if you do certain things that let the game know you are “interested” in aliens. It’s basically being in scientist career, searching for truth using telescope, owning space rocks or alien creatures (dead or alive) you can collect from space, space rocket on the lot, knowing alien, having daily contact and high friendship with an alien. From my experience only having contact with aliens and talking to them or being in relationship (friends/romance) is a guarantee that your sim will get abducted. My scientists only got abducted after aliens infiltrated the lab. As far as I know abduction is not connected to any world. If your sim do none of the above you will never be abducted. That’s how the game works for me right now.
Of course you can also increase the chance greatly by owning the satellite dish and calling aliens regularly.