How do you know when it's time?

We have a 15 year old mixed breed and have had her since she was 2. She has started pooping in the house about a month ago. She's definitely arthritic, partially blind and mostly deaf. She can start get around but her legs give out sometimes. She's still eating, but only with some meat mixed with her dog food. Today she peed on the floor and I found her just laying it, she didn't even realize. I feel awful putting her down and this is my first dog as an adult so it's hard to know when!