Well, it depends on the Warm Level you receive.
- Warm Level 1 will decrease a egg's steps from its normal rate by 5%. If I had an egg that needed 10000 steps to hatch, 5% from that would be 9500. Combined with Flame Body or Magma Armor, it turns into 4250 steps.
- Warm Level 2 will decrease an egg's steps by 30%. If I had an egg that needed 10000 steps to hatch, 30% from that would be 7000. Combined with Flame Body or Magma Armor, it turns into 3500 steps.
- Warm Level 3 will decrease an egg's steps by 5120. A 10000 stepper would drop to a 4880, and Flame Body / Magma Armor tones it down to 2440.
- Warm Level 4 will decrease an egg's steps by 7680. A 10000 step egg would drop to 2320, and Flame Body / Magma Armor bring it down to 1560.
- Warm Level 5 decreases an egg's steps by 10240. A 11000 step egg would now take 760 steps, and Flame Body / Magma Armor make it 380.
Source