At my age, very likely for life. I knew numerous people in my group therapy were on SSI for a long time. Once you get on it, there's like no checks to see if you're better. And autism is incurable, so I think I'm set forever. Plus, I'm moving into housing for specifically disabled people, it's not like they can just tell me to get out.
In the US, I think you have a great chance. Autism seems to be an automatic approval. You don't need anything about failed jobs and job interviews, they do a thorough investigation of your work history. If it's what you say, you're easily good there.
Warning that you'll probably be rejected your first try and need to appeal, that's just how the retarded system is. They try to draw it out and make you quit the process. I was lucky that I was accepted right away, probably because they thought I'd murder my therapist or something.