You can keep the ashes on your forehead for as long or as little as you want; there is no strict church rule about it.

The basic idea

Ashes on Ash Wednesday are a symbol of repentance and our mortality, not a sacrament with time limits. The spiritual “moment” happens when you receive them with the prayer (“Remember that you are dust…” or similar), not in how many hours they stay on.

Common practice

Most people do one of the following:

  • Keep them on until the end of Mass or service.
  • Wear them through the day as a quiet witness, then wash them off that evening.
  • Gently wipe them off earlier if they’re smudging, you have to wash your face, or it’s awkward for work/school.

All of these are considered acceptable in mainstream Christian practice.

How to decide for yourself

You can ask yourself why you’re keeping them on:

  • If it helps you remember, all day, “from dust you came and to dust you shall return,” you might keep them on longer.
  • If it feels like you’re “showing off” your piety, it’s fine—maybe even better—to wash them off.
  • If your job or environment makes it uncomfortable or impractical, you’re free to remove them without guilt.

A simple approach many follow: keep the ashes on for the rest of the liturgy and for some part of the day as a personal reminder, then wash your face normally whenever you need to.

Information gathered from public forums or data available on the internet and portrayed here.