When can I use an interrobang‽

Solution 1:

As far as I've seen, it's more an experimental thing than any sort of formally accepted marking. Thus, for informal writing where you can be sure that the reader/recipient knows your intent, it should be acceptable. For more formal writing or business communications, especially those where irony or sarcasm would be badly construed, it's best to be more cautious and make your intentions clear in another fashion.

That being said, I'd like for it to appear more often, so—especially online—feel free to spread it around.

Solution 2:

Short answer: no. Longer answer: at the risk of your reputation. Exclamation marks should be used sparingly anyway, and I for one tend to class interrobangs along with multiple exclamation marks as (to quote Terry Pratchett) a sign of mental instability.