Good stuff in this thread.
My girlfriend works as a BCBA for kids with autism. 45-75 day notice, based on her explanation, is common practice in her field as the company needs time to train and replace her. Otherwise, they lose the kids to other clients. She actually left her job today and gave a 45 day notice...
I recently learned my former boss at my first job gave a 6 month notice to start a new company so the old one can transition from her position. Company failed to replace her in time and the branch closed down.
Personally, I have done 2 weeks without issue. Did 1 month notice one time for a transition, and ended up going back to that employer for another job as I kept the relationship positive. Always look after number one, but understand the situation and observe the environment.