Open tabs
It seems it's taking rather too long for me to read these pages, so I'm going to post the links here and close the tabs.

Implementing your own recommender systems in Python by Agnes Jóhannsdóttir  519 days
Not even a valid link any more! I did read this, however I kept it open as I was hoping to apply some of the techniques at some point.  The Design and Implementation of Probabilistic Programming Languages  439 days

Game Theory (Stanford Encyclopedia of Philosophy)  340 days
I'm actually part way through this, the previous sections were open for 70, 24 and 635 days!  Problem Solving with Algorithms and Data Structures using Python — Problem Solving with Algorithms and Data Structures  249 days

GitHub  stared/sciencebasedgameslist: Sciencebased games  a collaborative list  141 days
Read through this but was keeping it open for reference  better to bookmark it! 
GitHub  aymericdamien/TensorFlowExamples: Concise TensorFlow Examples for Beginners with Latest APIs  52 days
Haven't started this yet, will actually leave it open, 7 weeks is only a baby tab.
These are outliers (the 99th percentile is 4 months), but that leaves 50 odd pages that were open for a couple of years and the worst for nearly 3.5 years!