Depends on your intentions 7:1 is recommended but usually in a crowded anchorage it ends up being more like 4:1 or 5:1, therefore I like to upsize anchor and length of chain to compensate for reduced scope. If we are talking anchoring for halibut 2:1 or 3:1 is fine.
3:1 works well for nice days. 7:1 if you are going by the book. 10:1 for storm conditions. Usually 3:1 will be just fine as long as you have an adequate shot of chain. A good rule of thumb is one foot of chain for every foot of boat length. Then I add a little more chain.