Agreed.

Confused...
Don't be. You have it correct.

What you're computing is the minimum time it takes for the RC sample/hold network to charge to the input voltage assuming you're making a full-scale voltage change. Depending on the accuracy you're trying to reach that normally takes about 7T-8T. Once it's charged up (or down), you're good to go, but it does no harm in waiting longer (as long as nothing else changes).

The only thing to watch out for is if you change PIC's be sure to check the specs of the new device. The source resistance of the internal switch and the cap can be different for different families, so that can change the calculation.