arpadveress 
Many times.
But to order it we intend to do once.
Very first you do based on EN/IEC 60079-10-1/-10-2. Inputs like:
- Simplified process description (including process flow diagram)
- List of hazardous materials
- Layout
Than we come to topics of design, realization, commissioning, start up. Furthermore we do maintenance, storage and other activities like waste water treatment.
All have their influence on Hazardous Area Classification.
Back to original question: many times we shall…
Keep up good work!
