It all depends on a number of factors
One thing that is important to remember is that a garden room is definitely far from the traditional shed. Maintenance should more like that for own house.
While a garden room might seem to be related to a shed, the origins of these structures are actually the same as traditional house building. Most of the materials and techniques found in house building are designed to last for decades and a good garden room will be the same. But we would suggest some annual inspection and maintenance to ensure your Henley last for years to come.