The instinct is to size a cleaning schedule from floor area, and it is the wrong starting point. Open desk space is the fastest part of any office to clean and the slowest to visibly degrade. What actually drives frequency is the small number of rooms that get heavy, repeated, unavoidable use: restrooms first, then the kitchen or break area, then whatever space visitors sit in.
That is why two offices of the same size can need genuinely different schedules. A twelve-person firm with two restrooms and a kitchen that gets used for real lunches will feel neglected on a weekly visit. A thirty-person office where most people are on site two days a week, with one restroom and a coffee machine, can be perfectly well served by the same weekly visit. Headcount matters less than how many people are physically in the building on an average day.
The second factor is who sees the space. An office that only its own staff use can absorb a longer gap, because everyone there has a tolerance for their own building. An office where a client might arrive on any afternoon cannot, because the cost of the space looking tired is not measured in cleanliness at all.