The Glance screen is a very nifty feature, allowing one to see important information without waking your phone, and is overall considered to be a net positive in terms of battery life, using only a few milliamps but saving a lot from not having to switch on your phone.
However in recent testing All About Windows Phone actually discovered the feature may be another major hidden battery drain in Windows 10 Mobile.
Testing the battery life on the Lumia 950 and Lumia 950 XL, they discovered that when the Glance screen is always on battery drain increased from 3.6% per hour to 6.3% per hour, meaning a phone without the Glance screen would last up to 12 hours longer on standby (16 vs 28 hours).
Now it is of note that by default the Glance screen is in Peek mode on the 950 series, meaning it is only on when your phone is out of your pocket or case with its face exposed, and for a maximum of 15 minutes, meaning for most users this is not a real issue.
If however you do leave your Glance screen always on and experience battery endurance issues, now may be a good time to change that habit.
Read the full testing methodology at AAWP here.