I had a 26 year save on FM 07 that saw me win the Champion's League with Rosenborg (Norway) 5 times, and the Norwegian league 18 years in a row.
It was a hard save to let go of. I fostered a thriving youth system to plug players into my custom 3-5-2 tactic, and I had some very memorable wins in Europe.
I distinctly remember playing Arsenal in the Champions League final for both my 500th and 1000th game (what are the odds of that). The 500th game was my first Champions League win, and the 1000th was my 5th. Towards the 'end' of the save, I was basically playing weakened squads in the Norwegian league, only putting out my 'true' starting 11 in Europe. If I put out my strongest in Norway, it wasn't unusual for games to end six or seven - nil.
My favorite player from the save was a South Korean named Young-Hoon Kim. He was an AM C, and I picked him up in 2012, which was the year I was allowed to start scouting the world. He played until 2027, and was integral in me winning three European cups. I also had a Portuguese youth product MC called Dias who played for me from 2008 to 2029, captaining for 17 years. He wasn't the best player, but he was consistent.
I'm pretty sure I put about 40 days worth of play time into that save, and I can still name most of my starting 11 from 2012-2022 off the top of my head, as this was my 'golden age'. I miss that save a lot. Since then, I've never really had the time to pump into newer versions of the game, so I tend to pick bigger teams with bigger budgets, as opposed to picking less glamorous European sides and leading them to a dominant position. I only buy odd-year FMs, and I usually go with Manchester United as my first save. Sometimes I try to start a Rosenborg save again, but it's so difficult to motivate myself to play with them without remembering all of the fantastic players I had in that 07 save...