In my experience...(3yrs off road monster trucks & 1yr onroad touring cars)
I have takeen an engine apart for the following reasons.
1. New engine, prior to break-in, to inspect the inside and verify no loose parts or "flash"
2. An engine that doesn't seem to be running well. I will inspect the fit of the piston and sleeve, look for scratches in the sleeve, inspect bearings and reseal the back plate and carburator.
3, The nitro season is over, and I will dissasmble, clean, oil and replace parts as needed.
Other than that, I do not beleive you need to take your engines apart. When I am done running for the day, I will disconnect the fuel line from the carburator and run the engine to burn off any excess full in the engine. I will then use about 6-10 drops of aftrun oil (fantom or hobbico), turn the engine over with out the glow plug and put the glow plug back in. I have done this on every engine I have ever owned, and I have never had rust in the engine or any of its parts.
I have owned a fantom 18, traxxas 2.5, sirio evo2 and picco engines. I have never broken a rod or crank. The only thing I have had happen to ruin an engine is inproper tune or a hi rpm runaway causing premature wear at the piston and sleeve.
I will say however, if you have the time, there is probably nothing wrong with dissasembling your engine to inspect regardless of duration. I would be careful, because each time you dissasmble your engine you can introduce dust and debris, gauge a sleeve, break a carburator or install the rod backwards.
I am posting this only so you have two contrasting views on the subject. Utilmatly the choice is yours, based on time restraints and how particular you are regarding your gear.
Finally with proper tune I would say most engines will last three gallons of fuel before loosing significant power, it really all depends on use and tune.
Doug