This varies and depending on your riding habits, you might be able to make do with longer charging time, or you’ll need to invest a little extra for something fast and convenient. If you ride at night a lot, then you’ll want a light that charges quickly. Most helmet lights will need three or four hours to fully charge, and the bigger the battery and longer the run time, the more time it will take to charge up the battery.
Make sure your light and charger are from a reputable source and conform to safety standards as, unfortunately, dodgy connections, batteries, and chargers can be a fire risk.