Are you asking about residential or commercial?
Residential, Every customer and carpet has different needs. I start cleaning. If the carpets are heavily soiled and the usage of the home would benefit cleaning more then twice a year. I would approach them about my maintenance program. I have 35 customers that fit that critiria right now, I clean for them every 3 months except for 1 customer I clean for every 6 weeks.
In a commercial setting during the estimate, I would bring up the importance of appearance management and when giving them the price for the 1 time leaning give also give them 1 or 2 options for maintenance cleanings.