Depends on what you are brewing. The old standard for regular gravity beers was 1/2/3--one week in primary, two weeks in secondary, three weeks in bottle. Often the exception happened more than the rule. These days the thinking is to just leave it in primary until it is finished (many feel 3 weeks is fine for average beers) and then rack to bottles and let it condition for three weeks.
Some will tell you "No less than one month in primary and then bottle." Others will tell you "Never, ever, ever transfer to secondary." Absolutes are seldom accurate and the nice thing about home brew is that it is yours and you can do whatever you want with it. I used secondaries for years and then decided to stop because it isn't often needed for things I brew. However, my brewing schedule has been such that I have had to move things into secondary just to make room in my larger fermenters.
When should you use a secondary?
1) When you need to secondary.
2) When you want to secondary.
When should you refrain from using a secondary and leave it in primary?
1) When you don't need to secondary.
2) When you don't want to secondary.
Just remember rule one comes first--the needs of the beer come first. We have to have priorities.