The polling interval cannot be less than the request-response cycle time, 

which includes the request transmission time, device response time and response message transmission time. 


The meter response can be as fast as 5 to 10 ms. 

Transmission via the Ethernet in both directions can take a couple milliseconds. 

If the master closes a connection every time after getting a response, then sometimes its requests may be rejected if they follow too frequently so there may not be available connection sockets in the meter. Anyway, 100 ms intervals look reasonable. 

For serial communications, the message transmission time can be calculated in milliseconds as follows: 10000/baud_rate * (message_length_in_bytes + 4).