When your chain needs lubing (and cleaning)
This isn't for the obsessive types, quite the opposite.
I've noticed a few things for when my chain needs lubing,
and if any of them apply IMO you should lube your chain :
(and your derailleur pulleys if applicable)
Generally in order of things getting worse :
1) It starts to makes more noise than nice and quiet
2) The cranks don't spin backwards as easily
3) Spinning backwards moves the derailleur cage
(Things carry on getting worse with no lube ...)
Some of this can be muck as well, I guess I attempt some cleaning
with WD40 as well as lubing 1 out of 2 or 1 out of 3 times depending
on the weather and what ground I've been riding through recently.
Spate of wet weather recently, and my folder went from 1) to 2)
without making the level of noise I'd normally lube it in good
weather, that avoids 2) happening. Live and learn I guess.
First really bad weather patch I've ridden it through.
Bike is covered in crap. The chain is oily pristine.
Keep your chain lubed ....... ;)