The capacity of a building dryer is stated in litres per 24 hours. As a starting point allow roughly 1 litre per 10 m³ for normal drying; double that figure for fresh screed, plasterwork or after water damage.
The stated figure applies at 30 °C and 80 % relative humidity. At 15 °C the same unit often reaches only half of it. Adding heat up to around 20 °C therefore delivers more drying capacity than a larger unit in a cold space.
Drying only works in a closed space. With windows and doors open you are dehumidifying the outside air and the meter stops moving.
Always combine it with air circulation: a fan sweeping along walls and floors carries moisture out of the material towards the dryer. Dryer, heater and fan together usually halve the drying time.
A 6 cm screed dries, under good conditions, at roughly one week per centimetre. Measure with a moisture meter rather than judging by eye.