I was lucky apparently - it only took me about three months to get a diagnosis.
But from what I've heard it can take a lot longer than that, mostly because of the shortage of specialist Autism centres. I know of someone on this site who had to travel across the pennines from Manchester to Sheffield to get a diagnosis! (Sheffield is just up the road from me so it was much easier to reach)
The underlying problem is that in the UK not just anyone can give a diagnosis of ASD, it has to come from an ASD specialist. This is actually a good thing because it means the diagnosis is more accurate and more informed. But it is also a problem because we don't have enough trained ASD specialists to cope with the workload.
In America, it seems any psychiatrist can give a diagnosis, so you can get seen a lot quicker if you have the money. But it also seems that there is a higher chance of misdiagnosis due to them not being fully versed in the condition.
Edit: By the way I'm a man, but that is largely irrelevant since sex has no bearing on how long you have to wait. You will probably be asked to bring a member of your family along to talk about what you were like as a child, as that has a big part to play. and then it will be mostly questions about how you cope with things in your life, how you perceive the world (ie sensory issues) they can get very in depth, but that is good because it means they are being thorough 
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Autistic dad to an autistic boy and loving it - its always fun in our house
I have Autism. My communication difficulties mean that I sometimes get words wrong, that what I mean is not what comes out.