2-3 months of the employee having been completely demotivated, festering resentment, and likely not doing anywhere close to expected productivity (much less overtime as the author was surprised about not getting).
"Out by 11AM" sucks, but it's _done_, minimizes suffering, and you can move on with life. Trust me.
The social (not legal) convention in the USA is 2 weeks (with 2 weeks pay in lieu of work if the "out by 11AM" happens). That's about enough to, if on good terms, wrap things up for all parties involved and transition accordingly.
Giving someone 2-3 months notice, and expecting performance as though it's going to proceed and end as if it were "out by 11AM" (to wit: work full enthusiastic hours for weeks on end with no distractions, then pack up and leave one morning) is absurdly unrealistic.
What about the suffering of the employee? They have bills to pay, rent or mortgage to pay, and food to buy. 2 weeks paid notice when you're fired means you have to scramble to get a job. The company have months of time to play for when to fire you, but you only get 2 weeks before you stop getting money? No way.
Of course not. I would prefer it to be around 2 weeks to 1 month.
But really, it depends on the job market. As an employee, let us say, a specialized laborer, I would prefer the notice period to be as long as possible because there aren't going to be any other jobs for me out there.
You'd really prefer "Clean out your things and exit the premises by 11AM" over 2 to 3 months notice?