I'm working on a plan to pay off my mortgage early. Each year I plan to
increase the amount I am paying by a fixed amount. For example, this year I
will pay $800. Starting next year I will pay $850, the year after that, $900,
and so on.

I know the NPER function to calculate the number of periods given a rate,
fixed payment, and present value.

How can I calculate the number of periods required to pay off the loan when
the payment is changing, assuming the timing and amount of the change are
known?

Thanks for your help.