I usually get to Krokotopia at around level 20. I do every side quest though.
My first run through the game, I didn't know I'd have to pay for basically the entire game, so I was stuck until I finally got around to paying for a membership. During that time I just fought many, many battles getting up to around level 16-17 before ever stepping foot into firecat alley or cyclops lane. By the time I reached KT I was already in my upper 20s.
I was around lvl 12 or so. It varied on all my different characters but i think i got to krok within lvl 10-15. I didn't go crazy on the sidequests because i knew there would be spell quests i wouldnt be able to do if i leveled up too much.