How long to develop? Depends on your objective really.
It takes seconds to park, days for a a good minisite/MFA/affiliate, weeks for a great minisite, and years for business ideas (if only due to capital req.). That's excluding time taken to SEO + rank
I have ideas for each of my domain, but coding and content and capital takes time.
There's a folder on my pc I created 23 June 2009. It has ~1+GB of documents, downloads I've collated since then with possible content articles, coding tutorials, and themes + plugins.
I don't think there is a policy, because a few blue-chip corporations owning generic domains have zilch on their domains.