The first show of the evening is packed. The later shows are easily accessible and less crowded. Since they replay the show about every 15-20min, it's easy to hit a later show.
There isn't a line per se, it is the whole area in front of the castle that you can hang out to watch the show from anywhere - in front of the shops, near the little stage, and also over on the bridge towards Kong (the bridge is least crowded). There is a good view from all over, since the lights are on Hogwarts up above everyone.
If the "viewing area" is too packed, they will stop people from entering the area until after the show ends and crowds disperse... then you can enter for the next show instead. This was the case for the first show of the evening. After the first few, it thinned out enough to walk freely throughout the area and get into FJ again (as that entrance is past the main show viewing area).
I'd just plan to arrive at least 40min after posted start time for the show, since they play it until closing time. Then you should miss the first 2 busiest shows.
My experience was from Feb, and it was busy in the parks.