Hi i use bottled water which has hardness of 28.7 mg/L or ppm of CaCO3. How often should i descale?

Given the low total hardness, that water is almost surely scale free.

Normally to estimate scale risk you need to know the hardness and the alkalinity, and when borderline or when dealing with artificially mineralized water it's best to know specifically the calcium hardness. Here's the bible for doing limescale estimation from hardness and alkalinity numbers: Jim Schulman's Insanely Long Water FAQ.