It can be different for everyone, depending on the issue you're working on. But most people typically report a reduction in their anxiety or stress after completing just one meditation.
Clinical trials have recorded significant reductions in cortisol (a major stress hormone in the body) just after a few minutes of tapping.
The key to tapping effectiveness is identifying and releasing the root cause of your distress, whether it be emotional, physical, mental, or energetic. The more specific you can be with your tapping, the faster you'll see results.