When to call it an Imperial

I understand where the term Imperial comes from but I was wondering when you can call your brew an Imperial "Insert style". I have a brown ale that is 9.2% and had a friend tell me it was an Imperial Brown Ale because of the high ABV. Is there a certain ABV you have to be at before you can call it an Imperial? What if the brew is high ABV, not hoppy, and not thick?

You just unwittingly touched on a pet peeve of mine....
The ONLY time "Imperial" should be used is in reference to a Russian Imperial Stout. Imperial is in the name due to who it was brewed for, not the gravity.
If a much higher gravity version of a style is brewed, and you want to designate it as such, just call it "strong" or "high gravity."
Sorry for the rant...

There are a number of US strong stouts labeled as "Imperial Stouts." These are generally +8% abv. The Southern Tier 22oz Blackwater series comes to mind.

It seems like "Imperial IPA" has slowly phased out and are now being labeled as "Double IPAs" These are generally also +8% abv or so.

I've never heard of a Brown Ale being that strong, but Baltic Porters are known to reach those levels. They can be brown colored but usually are exceptionally smooth for their alcohol levels. They are one of the hybrid ale/lager styles and authentic versions are usually made with a lager yeast but fermented at warmer temperatures, then lagered for a period.

The rate things are going (I.e. Near 6% abv "session" IPAs), it shouldn't be imperial until its over 10%.

/s

Seriously, use it when your beer is high gravity to the point that you want the drinker to recognize its high gravity. The term has lost any historical significance at this point.

Every beer has there own range of ABVs per style judged by the BJCP (e.g. IPA is 5.5-7.5%, stout is 4-5%, porter is 4-5.4%). As you get your ABVs above these values you can choose to call it Imperial. Imperial may have originally been part of a beer's name and who brewed it but has been accepted into homebrewing by 99% of people. Also, the dictionary lists one definition of Imperial as "of superior or unusual size or excellence". So, when you get to superior or unusual ABVs for the style, it is fitting to call them Imperial.