In regular vanilla I got to rank 11 on my rogue through a mix of solo and premades. On the private servers, which were massive, many, many people got to rank 14 solo. In all my experience… I would say you can absolutely get to 12 solo and by the time you get to rank 11 or 12 your server premades will likely spot you and approach you about teaming up while wondering what your stopping point is. 1 through 8 is easy. 8 to 10 goes fine as long as PVP is WHAT YOU DO while not raiding. 10 - 12 is going to be a long, dedicated month. 12 to 14 means you are unemployed.