The answer for me has always been – when I know for certain I’m leaving. If I applied for a job and didn’t get it, I would never tell my boss.
I’m rethinking that today for several reasons. I’ve found a job opening that, on paper at least, was written for me, and pays about 15% more than what I earn now. First of all, I’m not on a determined “get out of here any way I can” job. The position I’ll apply for is very close to what I described as my dream job in my interview for my current job. It’s a kind of “I owe it to myself to try for it” thing rather than of any deep disatisfaction with where I am now.
Anyway, I told a co-worker (who I trust to keep it confidential) about applying, and she strongly encourage me to tell our boss Liz. She said I owe Liz the courtesy, as she’s gone to bat for me several times. My co-worker also suggested another surprising thing, which is that I should use Liz as a reference.
I’ve never considered using my current boss as a reference for my next prospective job. Not only would know boss learn I was looking elsewhere even if I don’t get the job, it would just seem to be an uncomfortable situation to put her in.
My co-worker, who’s known my boss for 15 years, assures me that Liz will not take this as an insult, or retaliate in any way. She thinks Liz would want to help me get my dream job.