<<One I would be interested in if they ever recorded it would be average transmissions per minute or something like that as that may be a more accurate indicator of ATCO workload>>
When I left school in 1961 I was a lowly clerk in the Ministry of Aviation telecomms branch dealing with the provision of telephone circuits to ATC units. To determine the need for types of circuit Tels ran multi-channel traffic analyser equipment which measured the use of R/T and telephone lines for particular ATCD positions. One day my boss said he was amazed when they ran a check on GMC at Heathrow and found that 59 minutes out of 60 was R/T time! When in later life I worked that position I fully understood his amazement! However, there's a world of difference between talking to stuff on the ground and in the air in a busy TMA under radar control...
Lastly, if you want to hear really busy people... listen to Chevvron and his colleagues on a sunny summer Saturday afternoon. It'll blow your mind.