Monday would have been Dad's sixtieth birthday. I didn't even notice. It wasn't until Tuesday when I saw the reminder in my diary that I realised.
This then is the answer: three years. The question's changed a bit over time though.
In the days just after Dad died, he was constantly on my mind. Could I still picture his face? Would his voice fade from my memory? And above all, would the day come that I didn't think of him?
As time passed I got used to these thoughts rattling around in my head. After a while I realised: I still thought of Dad daily, but it was no longer an event; my memories of him were absorbed into my everyday thoughts. The day would never come when I didn't think of him, but the day would come when I didn't notice that I had. And that day was Monday.