When is the time to decant?

Generally it is better to allow too little time than too much. Very old, frail wines will start to oxidize within an hour, and will spoil quickly. Err on the side of caution - if when you serve a wine you think it needs more time to breathe, pour it out and it will change quickly in the glass.

The most convenient time to decant is probably just before your guests arrive, which normally seems to be about an hour before you want the wine. That will be enough for most wines, and should not be too dangerous for old ones.