The amount of Sage chakra that is absorbed is not unlimited and needs to be replenished at a constant rate. It is unlimited in nature, though. This means that if a user can constantly replenish their Sage chakra as they use it, they can stay in Sage mode indefinitely. Jiraiya did this through the two elder toads and presumably, Kabuto can use the snake that is attached to him. The reason that Naruto has a limit is that he uses Shadow Clones, which are not physically attached to him and although they are constantly absorbing natural energy, he has to disperse each clone in order to absorb the energy.
