IMHO.. you can get started ASAP..with NO knowledge what so ever..
however.. when you want to start doing things other thank blink LEDS and other simple projects.. its VERY VERY hard to get things moving! (again IMHO)
reading the internet and tutorials is great!.. but when you have questions.. (the monitor doesnt answer back!)
but asking for help is very hit or miss.. for example: you'll get alot of the same vet members posting.. but mostly people think you have the same knowledge as they do... so getting HELP is hard as its more terminology and datasheets..etc.. no real understanding that NEW people need 'help'..not more questions)
that being said.. answer are here/out there.. but some projects will take more time to work through
Its a fun and addicting hobby!.. just open your imagination to other ideas once you have an 'outlet' to make the ideas a reality.