I aim for a run of 5-15km between "locations" on my route. For example, one such "location" is simply where the line converges from double track to single track. Another such location is a passing loop with a disused station building and overgrown loading bank - ie, no fancy track layouts, just something different for the driver. At the moment I'm working on "Orange Hills", based mostly on the Orange Station/Yard/Junction area. This has been an interesting challenge after about 25km's or so of what is essentially just track and landscape.
As Trent says above me, keep it fresh. In recent weeks I've (over successive afternoons/evenings) laid about 6km of track from "Holloway" (Blayney) to "Reeves Forest" (Polonia Loop). I did the landscape, laid the track and then the loop. I did the signalling for the loop, and added some basic town items. I then got right into detailing the town, and textured the board to see what it would look like. I then textured a couple of boards back towards "Holloway" (closing the "untextured, undetailed gap").
When texturing got a bit old, I played a game - each night, I would detail and texture one board (I throw down all scenery save for the vegetation, texture, throw down trees/grasses etc, add more textures, and then tweak until finished). If I could finish one "board" sized area, I would then go on and do some more landscaping further on, lay track etc. Once I "closed the gap" I then built and put basic details down for "Springhill*" (Springhill) and then kept laying track to "Orange Hills" (Orange). Tonight I've thrown down points and signals, and a couple of speedboards. Tomorrow night I'll likely finish off the trackwork and start working on the town. When I'm done with the "Orange Hills" area, I'll work my way back to "Springhill" and "Reeves Forest".
I still have about 35km of track to lay, so that's an ace up my sleeve should I get bored again. Keep it fresh. The layout currently sits close to 100km of track, with about 20km still to detail, texture etc. I'm having too much fun to stop and release it (selfish, I know). I have a smallish module done independantly, but it's not much fun to drive as there is plenty of track, but not enough oppertunity for shunting.
Anyway, some food for thought.
Cheers,
(The other) Trent
*Okay, I couldn't think of a cool name, so just run with it.