Answer:
It all depends on how you use the Air Conditionning; we've had tenants who never used A/C, their electricity bill was under 300 baht most months; we've also had tenants that left the A/C on 24/7, with the doors and windows opened, their electricity bill was almost as high as their rent.

On average for normal usage (7/8 hours/night) depending on the temperature you want inside and the weather outside should range between 1,500 and 2,500 baht/month.
This may seem a bit higher than most smaller condos (30-40 sq-meters), our studios are 53 sq-meters with high ceilings (3+ meters); that makes for a lot more air to cool.