I’ve seen three experiences with getting approved for disability locally (USA, Nebraska), including my own, and they were all quite different.
My experience: Started having psych issues about 3 years into my last full-time job. Ended up taking FMLA multiple times; had to get documentation from my psychiatrist every time it was more than a couple of days. When I left that job, I spent the next two-plus years looking for a job, with no luck. Why I would think I could hold down a full-time job where recent experience had shown I clearly could not, I don’t know; I think I didn’t want to feel like a slacker.
I applied online for SSDI in May 2010. I was fortunate in that I had a lot of documentation, could provide detailed answers, and had a verifiable history of problems working due to my illness. Nevertheless, I got denied in September 2010, and contacted a local lawyer. He took my case, and at the end of May 2011, I was approved. I received my first SSDI check in September I believe, and my back-payment in November.
My mom’s experience: (A little fuzzy on this because it’s been a while.) She had a series of mini-strokes that culminated in a stroke that left her with considerable weakness on her left side, and an inability to do her job (nurse’s aide). She couldn’t even put on her plastic gloves the day she went in and tried to work. 
She was assigned a social worker through the Salvation Army, who helped her through the process, gathering all the documentation, talking to the doctors, etc. In the meantime, my mom moved in with me (and stayed). Finances were tight, but we didn’t have to wait long - within 3-4 months my mom was on SSDI.
My friend’s husband’s experience: He had a history of back problems, but not always the most thorough documentation. After a period of being out-of-work, their house went into foreclosure, and he attempted suicide. Come to find out, he’d been mentally ill for a while too (and not just as a reaction to being unemployed). Since that wasn’t something that had even been on his radar, at that point he had zero documentation for that.
First time he applied for SSDI, he got rejected. He may have applied a second time, got rejected. The third time, I believe, he got a lawyer, and was finally approved. It took him at least 2 to 3 years from initial application to approval.
Most people who have worked a significant portion of their lives will make $1,000-$1,300 a month on SSDI. My mom makes on the low end (she was stuck at a minimum-wage job for years), and I make on the high end (Bachelor’s Degree, office/admin work). Alone, we wouldn’t be able to comfortably make it. Together, we can afford the phone, basic internet, utilities, a cat, and an apartment in a slightly-sketchy neighborhood that’s beginning to gentrify. There’s stuff I miss (seemingly all my friends have smartphones, and/or iPad like devices, and go out to eat pretty often) but we get along okay. 
Best wishes and good luck!