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/science-based-games-list: Science-based games - a collaborative list - 141 days
 Read through this but was keeping it open for reference - better to bookmark it!
- 
    GitHub - aymericdamien/TensorFlow-Examples: 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!