The first question is, how dirty does it get ? On the residential side, we have never worn shoes in my house, kids weren’t allowed to eat in carpeted areas, no pets in residence and we vacuum pretty thoroughly – the carpets aren’t dirty and don’t need to be cleaned – never have been in 24 years. (They are however a very unfashionable ’80’s beige and pile)
In commercial cleaning world tho most people let their employees wear shoes, drink coffee at their desk, and so on tho, so eventually the carpets have to be cleaned.
Not all shoes are equally dirty however.
If employees walk in from a foundry, brick factory, metal bender, construction site, parking lot etc with oil, soot, dust, snow and mud on their shoes the carpet will get dirty quicker and need cleaning more often. Similarly in retail situations where thousands of people might walk in from a parking lot covered in snow, ice melting compounds, slush, or rain the carpet will get dirty faster.
That’s why Target stores have tile floors in the high traffic areas – carpet near the entries would get impossibly dirty within hours of opening.
That’s why factories, trucking companies, and so on also usually use tile floors or sealed concrete floors in areas close to doors used by production employees.
That’s also why we use entrance matting (see separate post).
So the first answer is – don’t even have carpet in situations where you are going to have to clean it frequently if you can avoid it – the appearance of the building and your costs will be better with some other type of surface.
Second, not all areas of the building get equally dirty. Hence the high soil areas need to be cleaned more often than the areas farther removed from entries and other sources of soil.
So the second answer is that you should schedule the cleaning of different parts of the building at different frequencies.
Third, winter is dirtier than summer in our corner of the world – so if you have a budget to clean the carpet twice a year you might do it in January and April (midwinter and end of winter) rather than in June and December (mid year and end of year.)
Once you get these concepts, the rest is just details.
For instance, in class “A” office buildings we often see a carpet cleaning program something like monthly cleaning of main floor lobby carpets during the winter, quarterly in the summer, weekly cleaning of any carpeted elevators where soil tends to get concentrated, monthly cleaning of any lunchroom carpets, quarterly cleaning of remaining common areas, traffic lanes cleaned once per year in the interior of the building, and on call or annual cleaning of offices and areas outside the traffic patterns.
A typical program for a single floor single occupant “B” building might be three cleanings of the lobby and lunchrooms in the winter, one or none in the summer, and a three year cycle for the office areas. The savings realized by only cleaning the low soil areas every three years pays for the more necessary and frequent cleaning of the high soil areas – a much better outcome than using the entire budget to do a once per year 100% cleaning for instance.
One can further modify (complicate ??) the schedule by adding less expensive “dry” or “bonnet” cleaning options in place of extraction for some of the interim cleanings. (see posts)
Just as an aside – I’ve never been able to satisfy myself that the extra carpet life you get from frequent cleaning of the less dirty areas saves you more money than it costs by delaying replacement of the carpet. Clearly the highly soiled areas need to be cleaned frequently for appearance reasons. Also soil is basically “gritty” or abrasive, and if not removed will tear the carpet fibers, accelerating wear.
Note also that frequent – daily or more than daily – vacuuming to remove the gritty stuff does more to prolong the carpets life than the annual “deep cleaning”, but once the grit gets to deeply imbedded for the vacuum to remove, “remedial” or “restorative” carpet cleaning is necessary.