Hi,
With a good connection to the database, 5 seconds or less should be fine. However, in practice, we’ve found that 5 seconds often leads to unnecessary switch overs, which is why we’ve been recommending higher lately.
There is one more option that was added recently (4.2.11, I believe) which could help: under redundancy settings, select “always keep project warm”. This avoids a lot of work involved in disconnecting and reconnecting to the OPC server when switch over occurs, which can help things run better. Depending on how much work it was doing, this can help quite a bit.
Regards,