Hi Hannah
My first attack lasted 8 hours, did they use any drugs to slow it down? Adenosine is the favourite, unless you've got asthma. It normally does the trick in no time.
There are loads of techniques you can use and you'll need to try a few out to see what works for you, but what used to do it for me was to (and I'm not joking now!) take a deep breath, hold it, close your mouth and push the breath against your cheeks without letting any air escape. After 10 seconds blow out rapidly. To increase the effect, lie on your back and raise your feet, prop them up against a wall. It increases the blood flow to your heart. Or get a thin straw, breath in and blow out through the straw.
When you're out and about you might look like a plonker! so you can also try splashing cold water on your face, drinking iced cold water, or lying on your side and tuck your knees up like a baby.
These techniques stimulate a nerve in your heart that helps to regulate the rate.
As for duration, you're young and I'm sure otherwise fit and well and your heart can cope with a lot, but speaking to colleagues in the medical field (I'm in the NHS) I always left it for an hour or so and then made it into the ED.
Often they will just monitor you, but when it's in a fast SVT rhythm it can take it out of you. Some people become dizzy and it's the best place to be.
You probably remember feeling really tired the day after.
Have a google for the valsava technique (that might be spelt wrong!) that's the clinical term for the techniques used.
Don't worry though Hannah. You might not have this again for 12 months or longer and if you do there's treatment you can have.
When you learn how to control it, you'll take it in your stride. At the moment it's unknown and frightening, but it won't hurt you, it's unpleasant but you've nothing to worry about.
That's a very long reply, but I hope it's helped!
All the best
Chris