The trail to Emerald Lake is 1.8 miles, making the out-and-back hike total 3.6 miles.
There are also options to extend the hike. For example, just before you reach Dream Lake, the trail forks with Dream Lake immediately to the right, and a 1.2 mile trail leads to another lake, Lake Haiyaha. This trail is much steeper and more strenuous, but offers fantastic views of the Rocky Mountains and the other lakes below.
The trail also connects to a network of trails between the Bear Lake and Glacier Gorge trailheads. If you really want to extend your adventure, you can head back via the Loch, and if you get an early enough and really want to get some miles in, you can even cut over to the Loch and head up to Sky Pond. There are lots of options to extend the adventure, as long as you plan ahead and know the geography of the area!