Really learning the trade... I gave myself 5 years to build up a decent home studio, and learning how to really lay great tracks. After that, I give myself 40 years to become a pro.
Humidity meter (!), Sonex Audio acoustic treatment, air filter
Additional notes
Don't start a home studio ! There is no way back, and you keep sending money to the studio gods each time you go in... Apart from that, it's both fun / art / and craft.