We self funded PopKey (a top 200 Utility app) as one of the first iOS keyboards. We waited until we were in the 6 figure range in users before we raised a dime. However, my business partner and I had to invest close to 1M of our own money to get it to where it is today. Very risky. I'm going to go against the grain here as a guy who has successfully bootstrapped an app and say if you can raise, raise fast and raise early. Just make sure you find right investors. They should be able to help you open the right doors. Also having some capital early will let you hire faster and get more momentum by buying users. Buying users matters more if you're in a winner takes all space and if it's a land grab.
We were lucky that we already had an in-house development team to leverage. We're also generating enough revenue now and have enough financial backing that we can continue to run PopKey independently for a long, long time.
Summary: if you want to go big raise early from the right investors.
My expertise, however is in bootstrapping, growth hacking, and lean start-up principles.