The actual time it takes to swtich between two multicast channels (sometimes called zap time) depends on a large number of factors. Some important ones are:
Generally, you need longer time (more buffers) to ensure higher robustness against jitter and variations in bit rate. This implies that with less buffering, you get faster zap times, but may experience audio/video problems if the buffer runs empty.
Starting from TV Software version 2.4, it is possible to modify the buffer used when playing video (or audio) streams. The default values used will ensure that the buffer never runs empty (or full) when watching typical MPEG streams.
On some streams where the characteristics is known beforehand it's possible to reduce the zap time by modifying the buffer size used. See the section on media player parameters for more details.