This is the ideal. Fish hold an average of 28 days and may spit as early as day 18.
On the day you notice her holding, start the filter for the fry tank on the main tank and leave the mom in the tank as long as she is not being harassed.
Two weeks later after beneficial bacteria has had a chance to grow on the new filter, set up the fry tank and move mom and filter in.
By two weeks if she spits the fry will probably survive and may even be mostly mature.
A fry tank is just like an adult tank except you add a filter intake guard (sponge) to prevent sucking up the fry.