They will never become unsafe to eat if stored in dry conditions.
The flavor will change over time - how long is appropriate is up to you, really. General rule of thumb is that if there's milk fat present, it will not keep as long. If they are packaged in a very high oxygen barrier packaging, they could be great for years (i've literally kept one brand for 7 years in sealed packaging and appropriate environmental conditions and it was great - very good).
If you plan to melt and coat something with the white chocolate - age is not going to be your friend from a rheology standpoint. Also, generally speaking, white chocolate is going to have a much shorter shelf life from a flavor perspective than it's darker counterparts. Personally i count the shelf life of almost every white chocolate in months. Most branded mfrs will, however, count their shelf life in the 1-1.5 yr range.