They should be worn 30-90 minutes before you want to fall asleep, depending on your individual biology. This answer may also vary depending on what other stimulants you've had earlier in the day (e.g. coffee or caffeine).
If you're a new parent or a shift worker, put on your TrueDark Twilights as soon as you want to induce rest. The key in these specific scenarios is to get the highest quality sleep even if you don't get a full 8 hours of rest.
TrueDark Twilights are also travel must-haves when crossing time zones. Airports and airplanes are notorious for having poor LED and fluorescent lighting. More importantly, you'll want to synchronize your body clock with the local time in your destination city. TrueDark Twilights can help you do just that, especially if you're taking a red eye flight. Whether you're traveling for business or pleasure, kick jet lag to the curb so that you can actually enjoy yourself when you get to where you're going!