All at once

This novel is set in a Chinese village before World War One.
Today is the protagonist Wang Lung's marriage day.
Here he refers to Wang Lung's father.

But he was not too forgetful to see Wang Lung dipping the water recklessly from the cauldron into a deep wooden tub. He lifted his head and stared at his son.
'Now there is water enough to bring a crop to fruit,' he said suddenly.
Wang Lung continued to dip the water to the last drop. He did not answer.
'Now then!' cried his father loudly.
'I have not washed my body all at once since the New Year,' said Wang Lung in a low voice.
<The Good Earth by Pearl S. Buck>
I'd like to know what "all at once" means here.
Thank you in advance for your help.