I am so sorry for your loss. It hurts so much. Time varies for each person and even can vary given the type of loss. My neighbors rescued a new Cocker less than two weeks after they lost their sweet girl to a very long, hard-fought illness. This dog is wonderful and has healed their hearts and given their other dog his youth back.
In my case, we lost our first dog to a lengthy illness (also hard-fought). She wasn't really old, but had been sick for a while. Our older dog missed her. He was nearly deaf and the only thing he could hear was her barks. We brought a young adult dog home three months after she passed. When our geriatric dog (14 1/2) passed last fall, we knew we wanted another dog, but it needed to be the right time. We asked our breeder to try to help find a dog compatible with our current dog. She contacted us before Christmas last year and told us that she was planning a breeding late spring/early summer and were we interested. The pup would be a 1/2 sibling to our dog. We took a spot on her list for a pup. He came home two weeks ago. So we waited over a year. It allowed us to have time to mourn our old dog, but also have some joy and anticipation for bringing home this new little guy. Our dog also had some time to adjust to losing her companion and being the top dog for a year. So, even in one family, we had two different ways to mourn.
Take time to mourn your beloved collie mix, but know that your heart is large enough to love again.
Added: I used this quote for my dog's memorial cards. I think it is appropriate to share with you:
"It came to me that every time I lose a dog they take a piece of my heart with them. And every new dog who comes into my life, gifts me with a piece of their heart. If I live long enough, all the components of my heart will be dog, and I will become as generous and loving as they are." ~Unknown