It's 3.5 miles one way, with a 2000ft elevation gain. So you are climbing up the entire time. Bring plenty of water and start early in the morning. As you want to be heading back down before high noon when the bugs come out. Your basically just doing switchbacks up the side of the mountain the entire time.
I honestly liked the Sheep Lake/Sour Dough Gap hike better. Which is on the other side of the mountain. You start at a higher altitude, so the elevation gain isn't as rough. You can stop at Sheep Lake, or, if your fit enough, climb up and threw Sour Dough Gap and get a great view of Crystal Lake. So you have the option of doing an easier or harder hike.
Also, though Sheep Lake isn't as big as Crystal Lake. The water is moving through it more quickly, so not as much standing water. Crystal Lake had WAY more bugs.
You also get a great view of Mount Rainier over Crystal Lake from the Sour Dough Gap hike. This home, the Mountis to your back, and covered by trees except for one small spot.