It’s exactly 600.

RD/SBD5-JP002 Star Battle Knight Asbolus
Level 7 LIGHT Fiend-Type Effect Monster
ATK 2400
DEF 1200
[Requirement] Change 1 face-up Attack Position monster (Level 5 or higher/Fiend) you control to face-up Defense Position.
[Effect] Choose 1 face-up monster on the field, and this turn, it gains 600 ATK.

Source