Top manufacturers/ brands recommend every 18 months should be a rule of thumb, we at New Finish Floor Care also recommend the same with an added recommendation. Due to some carpets getting more traffic and abuse than others and if that is the case in your household you should consider cleaning more often sometimes every 6 months. Remember waiting too long creates additional problems especially premature wear and tear and wear is not correctable.
No, not if it is done properly. Truckmount steam cleaning is the recommended and preferred method of cleaning by all the experts and Carpet manufacturers. With that said you need to hire a company that is experienced and educated on the proper ways of running the equipment, knowledge of the chemistry needed to clean without jeopardizing the warranty or performance of the carpet.