How do I housebreak my small adult dog?

Definitely keeping the dog on a schedule and routine, and lavishing with praise when they go appropriately are some of the keys to successful house training. You may need to crate train her when unattended in between bathroom breaks and when she exits crate, she's immediately taken to appropriate place to eliminate. Another option to a crate may be an indoor playpen or gated area (that is tall enough that she can't jump over) on an easily cleaned floor.

I'd suggest that most dogs generally need opportunity to eliminate every few hours. Even if they can hold it for 12 hours, to repeatedly have them do so, isn't best for optimal health. Likewise for us humans. Long term, I'd think there would be deteriorating health consequences (along the lines of urinary tract issues, infections, stones, and leading up to kidneys disease). So my recommendation would be to set up an area indoors where she's trained to relieve herself during those longer stretches if scheduling a walker or day/evening care isn't a possibility.