All the conditions mentioned for wuḍūʾ to be valid – such as the water being pure and not being usurped – are also conditions for ghusl to be valid. However, in ghusl, it is not necessary for the body to be washed from top to bottom. Furthermore, in sequential ghusl, it is not necessary for the body to be washed immediately after washing the head and neck; therefore, there is no problem if after one has washed his head and neck he waits and after some time he washes his body.
In fact, it is not necessary for the entire head and neck to be washed in one go; therefore, it is permitted, for example, for one to wash his head and after a while to wash his neck. Furthermore, if someone who cannot control the discharge of urine or faeces does not discharge urine or faeces for the length of time it takes him to perform ghusl and prayers, he must immediately perform ghusl then immediately perform prayers.