I ALWAYS wash my hands thoroughly before playing.
I ALWAYS pinch my wipe down cloth (clean cotton rag from some clothing article) around and rub down each string when done a playing session.
When i take a break I often wipe the cloth up and down across all strings from tailpiece to nut.
Often, I wash my hands again before resuming, and certainly if i was eating, or snuggling with the dogs - both have Lab blood and their coats get oily quickly or pretty quickly / a few days after baths.
But yeah, I start to notice the stretchy / bendyness character of the unwound strings change, and start to creep flat. Then I'll check for those flat spot divots. Then it certainly is time.
I dont really hear the dullness, though new strings sound a little brighter - which could mean I did not notice the relative dull sound of the old string, making it APPEAR the new strings sound bright.
I have a string change on deck today if my Musiclily tuners show up.
I will finally get some pics and a NGD thread on the SX Liquid over the weekend.