@Inyourwildestdreams
The quickest I can do a basic wedding (if for example, the couple has requested it to be quick, or they are running late or whatever) is 8 mins. That's no readings, no personal vows, no ring exchange, just in, do legal vows, legal bits, legal signing done. I did one of those the other day when the couple was half an hour late to their own wedding. For couples who are having a religious wedding elsewhere, they usually just want a very quick legal wedding, and the 'basic BASIC' wedding in that case is about 5 mins. Say some words, witnesses and couple sign a marriage schedule, leave.
A 'normal' basic wedding with no extra bits, but the legal vows/signing and ring exchange is really about 10-15 mins. Depends on what script the couple chooses. If its a long flowery one with extra paragraphs about what marriage means, it adds a couple of mins.
A good rule of thumb is everything all in, even WITH readings and personal vows, is done and dusted within 20 mins.
I would be surprised if you get to play 3 whole songs, usually it's a bit awkward if the walk in is done, the signing is done, and people are just sitting there waiting for music to end. I would usually signal to whoever is in charge of the music to wrap it up. Some people get a bit precious about 'which bit of the song they want to walk in to', and I think quite often they have envisaged themselves sweeping up a long church aisle to a particular chorus, but have forgotten they've booked a small room in the registry office and actually its only 6 steps from the door to the end.