When do you KNOW when it's time to put your dog to sleep?

I think dogs are almost indestructible due to their heart. My girl is 16 and a half on the 13th. She has a reoccuring infection on her hind, very bad. I got antibiotics and soothing antiseptics yesterday, but decided to take her in tomorrow to get her butt shaved, blood work and whatever else. i should ask, of course, about her quality of life.
She has been very happy and grinning until the last 5 days or so. Will the infection get better? She has trouble getting up. How bad does it bother her pain wise? is it time??