It's hard to give specific times, but they do follow a general pattern.
If you want daytime action, the RS40 is pretty reliable. It runs the Central Branch late morning/early afternoon, and switches Wellwood Siding and a couple of customers (stone, garbage, etc) south of Sunrise Highway. A good place to watch them is near the Grand Ave crossing in Lindenhurst.
The RS60 usually runs east from Pineaire, but I'm not too familiar with time estimates. Used to be a daytime run, then it was a night run, then it was a daytime run again...I'm sort of lost myself. There are other daytime runs on the mainline (extra stone trains and other stuff), but the movements aren't always predictable.
Night action is a little easier to track. The RS50 crew makes a round trip between Pineaire and Fresh Pond, usually leaving around 9-10 and coming back way later. The RS80 generally serves mainline customers in the Farmingdale/Wyandanch area, and usually arrives at Atlantic Coastal (C&D transload facility) on New Highway around 9:30-10:30.
Service on the Port Jeff and Montauk branches is as-needed and pretty sporadic, I wouldn't count on it.