Hi Ajtova, you should place your question at an entrance examination for college ;-)
The relative humidity depends on multiple factors. Solely the surface is not so important than rather the volume of your room > surface multiplied by the height of the room, to know what volume of air needs to be treated. However also the exchange rate of the air is a factor. This is affected i.e. by the frequency of opening the windows, is an air con or ventilation installed, the number of people in the room etc. As said so, it is possible that the device never manages to reach 60%. You state that the device reaches 50% however - I would go with that as the WHO suggests a rate between 40 and 60% - and you are already reaching 50%+.
https://www.constructionspecifier.com/scientists...
best, andi