When we are not using water for example we are away for a few days will the uv light produce too much heat because no water is flowing? When returning how long does light have to be on to kill bacteria?

No it will not get to warm. The UV lamp should be ON continuously. Repeatedly turning the lamp on and off will severely shorten the lamp life and
allow bacteria to pass through without being affected by the UV.

With that being said if you are going away for a week or so you can just turn it off. When you turn it back on it requires time to reach its full operating capacity. Allow the disinfection system to operate 3-5 minutes prior to use. In addition, open the faucet and allow water to run through the UV Disinfection System for 2-3 minutes to
clear any air or debris.

Sir, how do order replacement bulbs for this product?

I am sorry they where not showing in the store. Here is the link to it 6 GPM UV bulb

Sir, can this equipment be made to work for only 6 hours a day and switched of for another 18 hours continously for one year?

Sure you can use it for only 6 hours a day.