How long to find a long idea for interviews?

I assume that when you are already at a fund and looking to lateral, it's fairly straightforward to pitch an idea that you've actually come up with in the course of your day job. But as someone in investment banking looking to make a switch, I'm curious how long it should realistically take to generate a long idea for interviews. Is the norm to pitch a name that sits within your space? My hesitation on that front was MNPI issues, perceived or otherwise, but I would of course be able to generate a long idea in my sector much more quickly than outside of it.

To the extent that you were aiming to pitch something outside of your sector though, it seems like you'd have to wade through a lot of names that end up not being great candidates before you land on one that does make sense, and while some names could be eliminated pretty immediately, others would at least take a day or two of reading and research before you determine that they are not in fact great opportunities (if not longer).

Curious to hear how others in a similar situation have approached this, as well as general thoughts.