The navy originally calculated that the sailors would have enough air for at least two days.
But Captain Igor Dygalo, head of the Russian Navy's press service, said the revised estimate was made after talks by radio with the craft's crew. He said the sailors are doing well for now.
"Unfortunately the real situation now is that there is enough air only for 24 hours," Dygalo said. "The rescue operation will continue around the clock, without any breaks or interruptions."
"Audio contact has been established with crew members through a special underwater communication channel and there is an exchange of information under way," he added. "They feel fine. They are wearing warm clothes."
The 13-meter-long military submarine was taking part in rescue exercises off the Kamchatka Peninsula when it got stuck on the sea floor yesterday.
The navy said today it appears the craft's propeller is entangled in a fishing net at a depth of 190 meters.
Russia has asked for assistance from Japan,
which has dispatched several rescue craft. But Tokyo said earlier the boats would not reach the area until early Monday (8 August).