Training the parameters of a Restricted Boltzman machine
I believe the problem is that log-likelihood is not directly computable, because of exponential in number of units complexity. There exist different proxies to the true log-likelihood, for instance pseudo log-likelihood (more here), and in principle you can train RBM until PLL is not changed too much. 

Hovewer, there is a lot of randomness involved in the training process, so PLL most likely will be quite noisy (even true log-likelihood would be I believe).