The answer is going to depend on a number of factors. Some of which would be: type of radio used, firmware used, configuration used; this is for each node by the way.
With zigbee devices, there is always an upper limit of throughput. Once that limit is passed, the network becomes unresponsive. In many cases, it will recover eventually. but from an application or humans perspective, it will seem like it's 'locked up'.
Your question seems to imply that actual data is the only thing that needs to be considered. This is rarely true in the world of zigbee. There is usually quite a bit of overhead. It's not uncommon to see overhead outnumber data by a ratio of 20 times or more. There are a lot of retransmissions and route discoveries are expensive in terms of time and throughput lost.
Also, I get the impression that your end nodes will be sending data frequently to one point, probably the coordinator. If so, it needs to be mentioned that zigbee and 802.15.4 are a high latency, low datarate, battery friendly protocol. it is not intended to be used in a real time fasion. There are other protocols better suited for this such as wifi.
While you didn't mention which firmware you are running, I'm going to assume that you're using ZB as it's the most common.
In the end, the limits of frequency of a transmission are varied and depend on the situation. The more specific you can be, the more meaningful our answers become.