I feel your pain. My younger dog has SA and I am unable to go out at all during the week, except for 10-15 minute school runs, when I either leave him with a frozen kong and treat or pop him in the boot of the car with our other dog (for some reason he doesn't get upset when left in the car). It's taken a long time to get him to the point where I can do school runs without him crying and there's been a lot of two-steps forward, three steps back, for various reasons. He also used to cry whenever I left the room, but has finally got past that and this weekend I've been able to spend a few hours upstairs sorting out stuff to go to charity and he's been fine downstairs.
It's suffocating and not at all good for your mental health. Is there someone local that does dog-sitting? If so, you could arrange for them to come and sit with him for an hour or so while you get out for a bit.
Unfortunately there's no answer to how long it will take. It really does vary from dog to dog, based on lots of things from what caused the SA in the first place, the dog's personality etc. My older lad developed SA last year, when we lost our old girl to cancer. In his case it took 6 weeks of intensive work to get him to the point where we could leave him for an hour. We've now discovered he has something called Isolation Distress, which means he only gets upset when completely alone, as long as there's another body of some sort there with him, he's happy. (Hence us getting another dog, only to end up with one that has worse SA.
Older dog is now quite happy when left, but the pup - now 17 months - is ten times worse than he ever was.
)
Have you tried a thundershirt of a DAP diffuser or collar (dog appeasing pheromone)? Both/either would be worth giving a go.
I would also highly recommend reading the following two books, as they are really good for getting your head around SA, it's causes and treatments:
Don't Leave Me by Nicole Wilde
Treating Separation Anxiety in Dogs by Malena DeMartini Price