After extensive research and development ... and some creative math ...
the following results have been predicted for near optimal farm operation.
Basic assumptions:
you have all your crafting tools
steel farming tools
40+ slots worth of chests to hold inventory along the way
enough seeds to plant at least 6 full rows each of iron/copper/sand trees
seeds for 2+ rows of plastic trees
at least a dozen of each kind of gate seed
seeds for 2+ rows of pine trees
near optimal automation
It takes about 300 robot hours to farm the mats for an additional robot.
1 working robot = 300 hours
2 working robots = 150 hours
3 working robots = 100 hours
4 working robots = 75 hours
5 working robots = 60 hours
6 working robots = 50 hours
additional robots, beyond 6, do not provide a proportional increase in overall farm productivity