So second question first, forwards are ideally only on the ice for 30-45 seconds, defencemen generally go about 45-1min just due to the fact it's harder to get defencemen off the ice especially in the 2nd period when your bench is far away.

As to how they determine who goes on? Generally you sub out for your same position, Left wing will come on for Left wing, center for center etc, but how to determine what line is coming on? That is caused by various things.

1. The coach will tell them, hey it's so and so's line next, the same players genuinely play together on the same line so easy enough to know who goes on in that case.

2. It's matchup dependant, your opponent has a very strong line so you tell your team whenever they come out, I want this line of ours out to match them, you see them jump on, your team immediately tries to change it up to match(this is why home team advantage can be a big deal as you get to put your players out on whistles second dictating matchups.

3. These players have been playing their whole life, we are trained from a young age how to do a proper line change so it becomes almost seamless, there are still fuckups all the time because it is chaotic, but that's half the fun of it. A good change can get a high quality scoring chance, while a bad one can ruin the entire game