25 years for me. 1984 when some really fat kid smashed my head repeatedly into the wall outside our classroom. I learned three things from that:
1)Its not a good idea to get on someone's shoulders during a fight – they can easily drop you over their shoulders and then smash your head repeatedly into a wall
2) A tetanus jab in your arse hurts like hell and is quite embarrassing if the nurse administering said injection is your mate's mum who you've always had a crush on.
3) Being forced to play football the next day when you can barely walk because of said embarrassing injection in your arse also hurts like hell, especially when the other team all know of your pain and kick you in the arse at every opportunity.
Its no wonder I haven't had a tetanus since then.