As far as I'm aware it's a 24 hour Insulin, or near enough. Therefore it can be taken at any time, as long as you follow up the next dose 24 hours later.
I'm going to stick my neck out and say that the majority of those people on a single shot of Lantus tend to take it before bed and at the same time each day. Other people on a single dose find it better first thing in the morning. There's not really a wrong time of day so long as you keep the injection at around the same time each day.
When I used Lantus on a split dose, I used to inject twice daily at around 8am and again at 8pm.
A time suitable to yourself which is convenient and one where you'll least forget, I take mine before my evening meal although years before I have taken it before bedtime and even first thing in the morning, just remember to space it out 24 apart and inject within the same hour each day.