Pre-roll calculation

Hello!

In the documentation it is stated:

"Seeking in Opus requires decoding some pre-roll amount before playback 
to allow the internal state to converge (as if recovering from packet 
loss)."

I have encoded a RAW mono 48 kHz, 16 bit file using

     m_enc = opus_encoder_create(48000, 1, 
OPUS_APPLICATION_RESTRICTED_LOWDELAY, &err);
     opus_encoder_ctl(m_enc, OPUS_SET_BITRATE(48000));
     opus_encoder_ctl(m_enc, OPUS_SET_VBR(0));

I would like to ask how I should calculate the pre-roll. I used 1920 
bytes, but that is too slow for my purpose (I can not afford decoding so 
much pre-roll).
What is the minimum?

Or can that not be defined?

Thank you!

H. Weber