Daytime: Single flash modes last a long time and catch driver's eyes instantly. 400 lumens or more --- it'll be effective even at noon.

Night: 500 lumens or more. An easy control for adjusting brightness while riding is very helpful -- dial down as needed to extend the run times.

Lights that can run a lot more than an hour at 500-1000 lumens start getting expensive. Under an hour lights are tiny and affordable.

For night riding in the city with some traffic, I want 1000 lumens to compete with car headlights.