When can you use idioms?

You can use idioms any time you are writing informally. Informal writing includes such things as letters or emails to friends, works of fiction, blogs, and so forth. Formal writing would include academic work such as a master's thesis, legal documents, technical writing. If you are writing the instruction manual for a machine, idioms are probably not a good idea. Use your judgment, in any event. If you are writing for an audience which you believe has an understanding of idioms and which would appreciate the informality or color which they would add to your writing, then go ahead. I seldom use idioms when answering questions on this site. People want straight forward answers, nothing overly fancy.