Okbear said:
Get out of bed and do something quiet, drink tea, whatever, and only go back to bed when you think you have a shot at getting to sleep…it’s worked for me (sometimes).
[Memento qoute mode]Don’t believe his lies[/Mqm] 
But seriously normal tea has enough caffeine in it to keep me up for at least an hour unless I am really really tired. When I used to go to bed at 2-4am and get up at 12-1pm it was not a problem, but when I started going to bed at 10pm and getting up at 7am(job related) IT REALLY MADE MY LIFE MISERABLE until I figured out what the issue was. Very fortunately as soon as I stopped drinking tea at night my loud ass roommates took its place so the balance of things in the world was not seriously damaged 
But, there are some activities that I really enjoy using the bed for!
To address the OP, my time to sleep depends almost exclusively on the amount of sleep I got the night before:
More than six hours - about an hour to fall asleep, maybe up to three depending on how much sleep the day before.
Less than four hours - I’m out as soon as my head hits the pillow.
Between the two - about 20-30 minutes.
It doesn’t matter how tired I feel, how much I want to go to sleep, what time I go to bed, etc. It all comes down to how much sleep I had yesterday. I’m another one of those who replays the day in my head, sometimes in painfully minute detail.
I usually am sleepy when I go to bed. My eyes droop, my back hurts, and I yawn like anything. Then I slide underneath my comfy quilt and suddenly my eyes pop open and I think. I think about everthing.
Eventually I tell myself that I must sleep and try to empty my mind. That never works.
The best thing I have found is to count backwards from a thousand. Last time I made it to about 450 before falling off to sleep.
I don’t have a medical condition, and it seems very commonplace to me. Before I was taking Celexa I was up for at least two hours, sometimes long enough to see the sky lighten. After I started that anxiety medication the time shortened, and now it is usually between half an hour and two hours.
I am not looking forward to tonight. Last time I slept over here in the dorm it took four hours to fall asleep – finally did around two. Then I woke up at every hour. I always sat up and looked at the clock, and saw the clock at three, four, five, and six, and finally said hell with it and got up. The next day was hell; I had this scum on my eyes and my vision didn’t feel right.
Oh well. Gues it’s hard to understand unless you’ve been through it. Believe me, if I had any choice I would fall right asleep.