How long after I sign up can I start building my site?
If your account is automatically approved, your machine user will be created and ready for uploading within 15 minutes of you signing up! If your account is not automatically approved, it will be within at most 24 hours.
It will take a little bit longer for your domain name to actually work on the web though! New domain registrations and DNS transfers typically take 24-48 hours to work while the DNS propogates across the Internet.