IFR flight plan cancellation time

I had filed a flight plan for 8pm local the other night. I tried
amending the departure time as I was leaving 1.5 hours later than the
proposed time. I called the FSS and was told the IFR flight plan gets
deleted by the system 2 hours after the proposed departure time. I
thought it was 45 minutes or an hour. My friend who thought it was 2
hours. The only reference I can find says it varies from center to
center. Is this correct? Any real world experience?

And if this is in the Pilot-Controller handbook, it's time for me to buy
it and I apologize in advance.

thanks
Gerald