Mom wears depends but won't change them timely.

You don't indicate what your mother's cognitive state is. Could you have a heart-to-heart with her, telling her that others can smell the urine in the depends even if she can't, and to help her avoid embarrassment you'll give her a signal when she needs to change them?

Depends are expensive. Many older people are losing their sense of smell. And Depends are made to keep the wearer feeling dry. So, why should Mom throw out an expensive item when she doesn't perceive that it needs to be thrown out?

If you can possibly minimize the amount she goes in in her Depends, perhaps by a bathroom schedule as cwillie suggests, that would help a lot.

Honey, I've got news for you, people who forget to go to the bathroom are incontinent. Maybe the doc means the cause isn't something physical that can be fixed. Lots of older people are frugal and don't change as often as they should because they think the depends are too expensive, could this be part of the problem? Or perhaps she just doesn't realize she has gone. See if you can get her to use the bathroom on a schedule, even if she thinks she doesn't need to... after meals for example. And while she is in there she should be encouraged to change as needed.