End of life decisions

How do you know when it is time?

Winnie is 11 years old, I don't think it's time just yet but I can see her getting worse. Winnie has a hard time getting around, I can't walk her, or let her swim in the pool, I can't let her get worked up and exert energy otherwise she cant walk or even get up the next day. She is still happy, wagging her tail and gets excited when people come over, except now sometimes she doesnt even get up when people come over. She is on 3 differnt medications which were helping at first, metacam, gabapentin, and tramadol. However now they don't seem to be helping her like they use too, she is having a harder time getting around, I can see how stiff she is when she walks, I have two steps in the back yard and its hard for her to go up them. She pants A LOT, and just this morning she is constantly following me around, almost like she just cant get comfortable. She is still eating and drinking and like I said before she acts happy when she is awake, but she sleeps or stays on the dog bed most of the time.

How do you know when its time? I love this dog, she was my first and I have never had to deal with a dog getting old before. I don't want her to be in pain but I can't help but think even with the meds she is. I took her to the vet and they said I could try laser therapy, or acupuncture, but even getting her in the car is hard, shes about 80lbs, I'm strong and can lift her but it hurts her.

I just dont want to be one of those people who keeps a dog in constant pain because they dont want to let them go, but I love her so much and I dont want her to suffer. But I think she still has time, I just dont know what the signs would be when the time does come. How do you know?