I tend to rotate engines on and off the layout very frequently. That is, unless I have recently bought a new favorite engine; then it may stay on the track for a few run sessions. I do have two engines that get lots more run time than all the others though.
I don’t change the rolling stock much at all. I can’t remember the last time my inner loop didn’t have a 5 car passenger set on it. Sometimes I might add or change a freight car on the outer loop, but it is pretty much unchanged too.
I like Plasticville a lot, and have far more of it than I can put on the layout at one time. So every few months I go and change out a building or two. I also change the buildings on my tables for the modular club I am in nearly every time I set them up
I rotate engines after perhaps about 6 running sessions and I run train sessions almost every day. On my small 6’ x 9’ layout I have an engine yard with 12 engines and a transfer table so it is easy and fun to switch out engines. Rolling stock is mostly on shelves so I need to place them on the layout, but my rolling stock is constantly changing just like the engines. My layout, buildings, and track hasn’t changed since it took its final shape about 5 years ago. I do occasionally change vehicles or people.
I mainly run 4 trains sometimes will change out for a while but presently that’s it. I need to fix one engine and finish my 6th loop ( one that’s done is a around the room ceiling loop) The rest are either packed away or on shelves. a few have never been run but there on shelves at the moment. ( William steam girls set, Lionel NYC train set with command control ( one of the first tmcc sets)
I rotate all engines and cars. All are on display on shelves or on the setup. 10 engines, 75 rolling stock both passenger and freight. 50 autos, 200 people 25 accessories. Mixture of both modern and post war.
I just changed the setup after about 2 years. Added about 20 more square feet to it and switched to 072 and 054 curves, eliminating almost all of my Super O track.
I have a “time table” and I run certain types of trains (eg through freight, local freight, inter city freight, hot shot passenger, local mixed train etc) each train runs at a certain “time” of the day and I adjust lighting conditions to accomodate the time of the day (night effects with black light and paint that glows in black light). Only certain engines are used on certain types of trains, and the consist varies with each run depending on the destinations of each car (the chemical plant always gets tank cars and covered hoppers, the packing plant always gets reefers and stock cars etc) .
Asside from adding scenic details as the mood strikes me, after every three trains run, I change the cars and trucks on the layout and I have over 200 that I rotate in and out of certain locations (oil trucks go to the refinery, cement mixers and dump trucks go to the cement plant etc).and, of course, I actively switch the rolling stock at the various sidings.
I have a “script” that I follow when I run trains and a card routing system for rolling stock that I simplified from what the “serious model railroaders” use. I guess I am inspired more by Frank Ellison than by Tony Koester.
What I do would not suit everyone, but I have had the same layout for 25 years (and have operated it this way for all 25 of them) and it has kept me amused, busy and satisfied. I can’t ask for more out of a hobby.