I recently stumbled across a study with truly long-term follow-up. Fava and his colleagues (2001) collected follow-up data on a sample of 132 patients treated for panic disorder and agoraphobia. The subjects were panic-free following 12 sessions of exposure therapy and were re-assessed 2 to 14 years following treatment (mean 8 years). According to the summary I came across, 23% had a relapse during the follow-up period while 62.1% remained in remission after 10 years. Relapse rates were higher for subjects who were younger, were more depressed, had residual agoraphobic avoidance at the close of treatment, or who were using medication concurrently with CBT.

For details see:

Fava, G. A., Rafanelli, C. Grandi, S., Conti, s., Ruini, C., Magelli, L., & Belluardo, P. (2001). Long-term outcome of panic disorder with agoraphobia treated by exposure. Psychological Medicine, 31, 891-898.