I believe this is not implemented, as stated on the manual.
Probably a forgotten part of this project.
Would be nice to have it done. Would allow me to use the integrated captive portal of ZS. Mobile devices nowadays hunt so much for bandwidth that this is a very important feature to enable.
For each class of accounting is possible to define an upper limit in Megabytes for the traffic generated and the maximum number of hours or fractions of hours for which the user can remain connected. When the user exceeds at least one of the two limits is automatically disconnected and no longer allowed to connect until the counters are not reset. Note that the bandwidth limit in Megabytes per second has not yet been implemented in the current version of Zeroshell (1.0.beta16).
I just tried with with v3.8.2 and it is not limiting.
Kudos to Fulvio for putting this project together by the way, I keep coming back to ZS after trying other solutions.