Ok, you do not provide much information, So I am going to make some assumptions about your code.
int err, got_frame;
AVFormatContext *avctx;
AVPacket avpkt;
AVFrame *frame;
// You open file, initialize structures here
// You read packet here using av_read_frame()
{
AVStream *stream = avctx->streams[avpkt.stream_index];
if ( 0 > ( err = avcodec_decode_video2 ( stream->codec, frame, &got_frame, &avpkt ) && got_frame ) )
{
int64_t pts = av_frame_get_best_effort_timestamp ( frame );
// TODO test for AV_NOPTS_VALUE
pts = av_rescale_q ( pts, stream->time_base, AV_TIME_BASE_Q );
// pts is now in microseconds.
}
}