If Sheldon Brown starts fondly reminiscing about how much he loved riding that model, then you've probably been waiting too long.
If the LBS can no longer get spares for that model, then you've probably been waiting too long.
If the oldest mechanic in the LBS has never seen that model before except in an old catalog, you've probably been waiting too long.
If you visit the smithsonian museum and see that model as part of the "history of cycling through the ages" display then you've probably been waiting too long.
If an archaeological dig finds a fossilised version of your model, you've probably been waiting to long.
If the bikes' manufacturing process involves a hammer and anvil, or tapping rubber trees, you've probably been waiting too long.
personally, I think 2 weeks would be my limit.