My first - waters broke at 3.30am, told to go into hospital and labour started on the way. Contractions every 3 minutes at 5.30am, not dilated at all - the midwife said that they would settle down, as it was because my waters had broken. 6.30am she declared I was about to have a baby and all hell broke loose. DS born at 7.30am. 3.25hr labour inc placenta, according to notes.
Second twins at 35 wks - started to feel something (non hurty but knew the feeling) recognisable as contractions at 2.30am after going to the loo. Tried to go back to sleep but woke DH at 4.30am when they were definitely every 3 minutes and he rang the hospital who would only speak to me (mid-pre-labour-crap on the loo). I told them I was 35 weeks with twins and having contractions every 3 minutes and they told me to come in at 9am. I told her that I had seen a doctor the day before who had told me to come in on the first contraction (he was the only person to take my worries of a fast labour seriously). I told her that I had been having contractions for 2 hours already so I was coming in now. By the time we had woken a poor Dfriend up to take DS, it was 5.30am when we got in. Put on monitors, given pointless IV line by junior doctor 'in case we need to speed things up' and waters broken on instructions of registrar (in bed). Declared 4cm by JD after much fumbling.
At 7am, I though 'God, this is boring' had some gas and air for fun (missed out on it the first time) then decided to go to the loo and finish what I had started before the phone call. After they had disconnected me (and poor DD1's head) from the equipment, I had a big contraction that turned out to be pretty painless, went to the loo had a wee, then massive huge contraction, got back to the bed, DD1 mostly born on her own, whilst mw getting backup and rubber gloves, DD2 born in stirrups in the delivery room, with a room full of people who should have been there. However, she was an absolute star, turned round and came out head first by herself, so they were not needed. 7.30am again.
I was told afterwards that if you come in in the early hours, they make you comfortable and pass you on to the day staff. Explains a lot. 
I think the second was slightly longer than the first (a million times less painful) but it depends on the definition of when labour starts. I don't think I ever fulfilled my hospital's criteria on what is defined as 'in labour', before I gave birth, both times.
My advice, be prepared, and be prepared to be assertive. (Don't expect them to have read your notes from the first time either!)