Matthew, if there are any K-9 handlers reading this post, you'll probably get a wide variety of answers.
One thing you didn't mention was and how many legs for each track. If you are fortunate enough to have no snow in Dec, (we have lots) I would vary the length of the tracks as well.
If you are returning to work after days off, I would start with a short track (200 meters) maybe 15-20 mins old. (of course you can vary these slightly depending on your dog) This will ease the dog back into things after 3-4 days off. One thing I should mention, is that our dogs are always kenneled and they live outside. So on days off, we want them to be bored, they still get sufficient exercise but little play time, this is very important. Theory is, when they return to work, they are raring to go.
If you have short grass, perhaps a farmers field once a week, I would do a longer track about 1000 meters with 4 articles, aged about 45-60 mins. This will keep up the dogs stamina (and yours) and the scented artices will build his drive. When our dogs indicate on an article, we have a quick tug o' war then continue the track. We keep it very short so they know that it is not the end. And always end the last leg of the track on a down wind, to keep the dogs nose down.
You can vary the end of the track with a toy, tug, or a bite. What works nice too, is to have a quarry lay the first 2 legs of a track then stop at the 2nd leg. When the quarry/decoy sees the handler start the track have the quarry continue to lay the remainder. Watch your dog at the end of the second leg, as it will be a "hot spot." When he carrys on, you should notice a difference in his tracking drive. (similar to a suspect bedding down for a bit, then continue to run - the area will be full of scent)
After a negative track, a real track where you were unsucccessful at locating a suspect, I would do at least two short training tracks. You always want to end on a postive with the dog.
The "easy - hard principal" is a good one to remember. If the conditions are poor, try to make it a little easier for the dog. For example if the track is aged 40 mins, and the weather is terrible, double or triple lay the first 10 meters to ensure the dog has a good start.
At the end of your shift, I would give the dog an easy one, as he's probably pretty tired after 4-5 days and he gets to end the week on a good note.
I think your training schedule will also depend on how busy you actually are at work, as you don't want the dog to shut down.
We also do a lot of training in industrial parks, as the distractions are usually minimal. Its also a good place to do some unknown tracks if you have a quarry.
Our unit trains every two weeks, an unknown tracking scenerio would be set up in a residential area, you would respond as you would on the street and attempt the track. The track usually goes thru yards, back lanes, over fences, road crossings, etc. The person setting up the scenerio would accompany the handler to minimize errors. This one is mostly for the handlers to get them to think as well. The biggest challenge is to re-aquire the track after the negative. We try to make it as real as it would be on the street.
I think the key is variety, tracks, area searches, small article searches, building searches, etc it enourages the dog to use the nose.
On a side note, when we train a green dog, the first 5-6 weeks is only tracking and obeidence. We want to lay the foundation - to keep his nose deep and down. When you introduce other searches to early, the dog trys to air scent, thus making it difficult to keep his head down latter.
Try doing your small article searchs on hard surface. The dog has no choice but to keep his nose down. I'd really scent up the articles, maybe a recently fired casing. And try to have someone else lay out the small articles, as the dog will sometimes seek out your scent when searching for the articles.
Think outside the box. Hope it helps a bit.
To a dog, the whole world is a smell.