Why does your heater take so long to blow hot in 1999 expedition?

Answer 1

Low coolant level, clogged heater core, thermostat stuck open.

Additionally that model had different size radiators available the larger the capacity the more difficult to heat up. You may also try covering a portion of the radiator/front grill, this will reduce the airflow across the radiator and will assist the heat retention. If you don't wish to go thew expense of a grill cover use a piece of cardboard between the grill and the radiator secure it in at least two points.