Its not so much a matter of how much string you cut off - its how much string you leave to wrap around the tuner. The high E should have about 5 or 6 full turns on the peg when its tuned to pitch. The low E should wrap around twice. The rest go somewhere between those numbers, depending on the string thickness. The idea is to get just one layer of string wrapped onto the post. If there's not enough string wrapped around the post, it will slip out ot tune. If theres' too much string on there, that can cause slippage too. And it looks gawdawful. After you've done it a few times, you'll get pretty good at knowing how much string length to allow.

Meanwhile, congrats on your first time! :)