I had my second abscess (first was drained) and it took a while to get into see my surgeon to get it drained. I was taking sitz baths 3-4 times each day and the abscess finally opened on its own. I still went to see the surgeon because I knew mine was connected to a fistula and needed a cleaning and a seton. But, they can open on their own in time. They will not be cured, especially not a large as you describe, with antiobiotics alone. However the antibiotcs should help keep the infection in control and fevers down.
Best of luck! Sorry you are going through this!
--40 year old female, dx as UC in '04 (1st symptoms in '03), switched to Crohn's in '05, 1 fistula, crohn's colitis, limited to large intestine
--rejected (reaction/didn't work): Asacol, AZA, 6-MP, MTX, Remicade, Humira, prednisone
--stuck on methylprednisolone, tried Prochymal in Phase III study (can't wait til it's approved!)
--started Tysabri 3/21/08
--single mom to 10-year-old girl