Still not 100% sure if I should upgrade or not, but got pre-approval for a 12 Pro Max 128GB anyway.
Currently rocking a 64GB X. Got it in March 2018 for £850 iirc, so it's around 2.5 years old (3 year old hardware however) & it's randomly crashing when under somewhat heavy use (switching between several apps, FaceTime, Snapchat etc). Also sometimes happens when I plug my phone in at night, mount to a wireless charger during the day or plug into CarPlay.
Performance-wise, it's an absolute trooper and doesn't feel slow at all. Battery life is not great however.
I like to keep my phones for at least 3 years. I kept my iPhone 4 for 4 years & iPhone 6 for 3.5 years (I did replace parts in both of these several times). My iPhone X could be the shortest one yet.