In practice, most signs last over 10 years outdoors. But, sign durability is a complicated topic!
Signs that face north last longer than sign that face south. Signs subject to constant water (such as dew), tend to fade a bit faster than signs that don’t have the wet-dry cycle. Signs that are made from premium materials, such as 3M High Intensity Grade or 3M Diamond Grade have a longer reflecttive life than signs made from 3M Engineer Grade.
There is also a good deal of variation in how durability is measured. For most applications, a sign is durable if the print is readable and instructions can be read and obeyed. For critical highway applications, a sign is durable only if it’s reflective values are above a certain baseline.
In practice, we have seen refelective traffic signs of ours that still look good after 20 year outside in some places. In other cases, vandals have taken their toll and the sign is not longer readable.
The cost to install a sign is equal – and in most cases, far greater – than the cost of the sign itself. Adding a few more years of outdoor life make compelling economic sense.