Hi Manisha; it really depends on what kind of orchids you're trying to grow and how bright the existing lighting is. In general, growing under lights, it'll be 8-12 hours per day of lights being on.
If you'd like to assess if your current growing area has enough light, have a look at this post:
Orchids on a Balcony: New Gadget: Light Meter
It gives you rough guide to how much light you need for various common types of orchids and methods to work out light levels (without a light meter) lower down. There are lots of cellphone apps too.