Many people have heard that red wines, like merlot, should be served at room temperature or even "warm." However, wine is a very old drink and this rule refers to room temperature in the days before central heating. A merlot should be served at around 60 degrees Fahrenheit or so, cooler than a centrally heated 72.
In an Ice Bucket
Water conducts cold very quickly, so a warm bottle of merlot in an ice bucket will chill in about 10 or 15 minutes. Upend the bottle once, to chill the top half, too.
In a Refrigerator
Air conducts cold more slowly than water, so a bottle of merlot in the refrigerator may need an hour or two to chill. If it is completely chilled, take the bottle out of the fridge about an hour before you plan to serve it.
Overchilled?
If you have overchilled the wine, pouring it into a glass will warm up a small portion in five or 10 minutes. Cupping your hands around the bowl of the glass will also help warm the wine.
Too warm
A red wine that is too warm will taste harsh and sharp. Warmth accentuates a red wine's tannins and acids.
Too cold
A red wine served too cold will taste dull and chalky. It will also be difficult to smell any of its fruit aromas.
References
Fear of Wine; Leslie Brenner; 1995