Traditionally, your BG's lose 1 hp a night, every night, until they reach 20 and then it bottoms out and they no longer lose any more except through getting attacked.

With the latest change of linking the BG's health to their performance, our number crunching shows us that it would changed the new floor from 20 hp to 60 hp.  This means that your BG will lose 1 hp a night, and if never trained, will now only stop at 60, rather than 20.