how long can I stay in korea on visitor visas

I think (but I'm not 100% sure) you can get another and another 90 days in Korea without visa. BUT
"on the condition that they do not engage in remunerative activities during your stay." HOW you can prove this is another issue. It's probable that they will question you about your long stay in Korea. You need to PROVE that you can support yourself without workong for other 3 months -+ you need a ticket out of Korea. I don't know the amount of cash they require for 3 months but it's defined.
Luckilly you don't need a visa -because in that case- you need to submit your certificate of employment, personal bank statement (in the right form) etc etc.

Luckilly you don't need a visa -because in that case- you need to submit your certificate of employment, personal bank statement (in the right form) etc etc.

and even in this case they can refuse you an enty