There is not really any hard settings that I have found.. We are using server and serving 200+ iPads off a 10MB fiber connection... In the process of upgrading to 50MB though.. 10MB is not quite enough for us.. I just monitored the connection for a few weeks, then based on the data decided to upgrade..
The design of your data model, size of your found set and the design of the layouts used with the remote client all can have a very significant impact on system responsiveness. Design choices that minimize these variables, when possible, can then also significantly impact the results that your clients get.
And I've always wondered how much, if any improvement might arise from Convert to Seperation Model with the interface file local to the remote client while the data file remains hosted from the remote host. In theory, that reduces some of the data that has to be served across the net to the client as such things as layout graphics, etc are now locally sourced from the client machine instead of the remote host.
But I've never had the opportunity to test that idea in FileMaker...