How much
time can I spend at airport "b" and still count this flying as
cross-country time for the purpose of meeting the aeronautical
experience requirements for an instrument rating?


As much as you want. It can even be overnight.

There are no rules about what constitutes a "flight". In the FAA FAQ they give
several examples of "flights" that last several days with stopovers. However,
in interpreting the regs, they look to the "purpose of the flight". The FSDO
may determine that if you stay long enough to require re-planning and
re-briefing the next leg, that the cross-country-ishness of the flight (for the
purpose of meeting this reg) may not apply. But this is just speculation on my
part as to how they may interpret the regs. It is not actuallly =in= the regs.

Jose

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