Six months is way to high.
I think you're being compared to someone with director level responsibilities or someone has been in the company for more than 15 years.
Even at that level, six months is generous. Three months, tops to ensure transition. One month is all that is required. It really depends on the status of the current work threads.
Haha, yeah. Everywhere I've ever worked and resigned from, all but one have been 3 months notice. Even after raising with the issue of knowledge transfer with them early on, usually it gets ignored until the last 2 weeks and then there's a massive panic because there isn't enough time.
Best solution if you're D+ level is to assist in finding your replacement, train them and introduce them to the team before your departure. Be strategic about it. More importantly, be empathetic to those who granted you the authority to lead them, your team(s), and make sure they are in good hands. Don't abandon them. Make sure loose ends are either tied or handed off to capable hands.