"When do I come in?"
Translation:J'entre quand ?
I think there are only to viable options: "Quand est-ce que j'entre ?" and "J'entre quand ?"
Subject-verb inversion would be "Quand entré-je ?", but it's not used because it sounds really awkward. In fact, inversion is used with je much less often than with other pronouns because of this (and also because it often sounds overly formal).
I was just marked correct for "Quand entré je" although I doubted I was right.
Not sure why there is an accent over the "e" at the end of "entré" though.
There is another extensive discussion thread around this sentence, but it hasn't really fully answered the many queries.
The accent is added to make the word pronounceable: otherwise "entre-je" would have too many consonants at the end.
But adding this accent is really un unnatural thing to do even for us French, and I can guarantee the vast majority of native speakers don't even know about this rule (I found out about it not that long ago when a friend asked me what the inversion of "je mange" would be, so I looked it up and was surprised to discover it was "mangé-je").
So although this type of inversion is accepted on Duolingo as it is technically correct, it's important to know that nobody ever uses it becausd it's too awkward sounding.
The most correct "Quand entre-je ?" but it's seldomly used.
Most of the time you will ask when you CAN or when you MUST: "Quand puis-je entrer ?" or "Quand dois-je entrer ?"
As for the form with "est-ce que" it should only be used when you don't feel using the inversion form (verb-subject) which may, just like in this case, sound properly.