It would depend on what it’s made with. The driftwood would last for years, I think, but glue or ribbons or whatever not so much. Also, if the door is covered by a porch or whatever. :)
If its strickly drift wood, then it can last years. It already took years for the driftwood to become smooth. You could also cover it with a clear varnish to help protect from weather.
Hi Janet: A lot depends on what it's subjected to weather wise, rain, snow, sun, wind, etc. If nothing is effecting it, it should last a good long while, years, even ;)
Is it covered? How is the driftwood attached? Driftwood itself should weather well. Glue however might breakdown.