The size of the sinking fund depends on the scheme, it's size, facilities and materials used in the construction. For example, if the scheme has lifts, the boxes and engines may need to be replaced / overhalled every 8-10 years, the building may need to be repainted every 4-5 years. The key to the amount of the sinking funds is it's correlation to expected future costs for the development in question. Unless management companies have researched and planned for these future events with realistic budgets, the occupiers will be hit for higher charges in the future or the work will not be done which will probably negatively impact resale value.
Take a walk around a 10 year old scheme and see if the grounds have been maintained, painting is done, carpets replaced etc. This will give you a good idea of the dilligence of the Management company.
Without knowing what work has been done using the sinking fund or what work is planned there is no way of knowing if it is adequate.