I've been working on https://kernl.us for a little over 2 years now.

It's currently making $650 / month and growing.

Something I "knew" but never really understood about growing a business is how long it can take. Granted this is completely on the side, but it still takes a long time to find your market, grow, get customers, etc. Learning that this is actually a marathon was valuable.

The other thing I have to actively manage is building things that are valuable -vs- things that I find interesting. As a developer I LOVE tinkering with new technology, but when I'm wearing my business hat upgrading to the new hot frontend framework doesn't make that much sense.