Manufacturers will indicate how long the light should last between charges. These times are quite often optimistic 'theoretical' best case scenarios which don't mention your lights may be brown in the last 15 minutes of use or, performance as the battery ages. You don't usually find that stuff out until you read the rest of the owners manual. So, read the owners manual before you buy. Rechargeable batteries come in many varieties and they all eventually die.
Premature death is usually the result of improper use and maintenance.
Brand new, a 7Ah battery will last longer than a 4Ah. New technology batteries weigh less, charge faster, cycle deeper, last longer cost more etc. I bet you'll find, as I have for commuting, you won't purposely diffuse the light for ambiance. Off road it is nice to have ambient light for low branches, but this effect can be created with two lights aimed high and low. I wouldn't ride with one light anymore.