How much should I reduce food after spaying/neutering?

Rather than an arbitrary percentage cut, recalculate your pet's calorie target using their current weight and activity level, since a calculator will account for the lower metabolic need more precisely than a flat percentage rule.

When should I start feeding the adjusted amount?

The metabolic change is generally considered to take effect within weeks after the procedure, so it's reasonable to reassess calorie needs and body condition within the first month or two post-surgery.

Does this apply to both male and female pets?

Yes -- the metabolic rate reduction has been documented in both neutered males and spayed females, in both dogs and cats.

Neutered/Spayed vs. Intact Pets: How Calorie Needs Actually Change | PetCalculator

Spaying and neutering measurably lowers a pet's calorie needs, which is exactly why so many pets gain weight after the procedure without a diet adjustment. Here's the comparison and what to do about it.