It is 2010. I'm sure there are apps that were a success after 1 day, 1 week, 1 month, and some after a year or two of pivoting.

If its an innovative and useful idea users "get it" almost immediately.

If it's not as innovative and you're building an app that is improving another idea it will take more time for users to see it, try it, and switch.

I don't think there is a magic number.