If you use ELM322 and ELM320 based interfaces with ScanTool.net software v1.06, you will get a maximum refresh rate of about 4 samples per second. ELM323 v2.0 gives you about 2.5 samples per second.
The data rate for J1850 VPW and ISO9141-2 is 10.4kbps, and four times as high for J1850 PWM (44kbps). The maximum message length is limited to 12 bytes (for both requests and responses), which means that theoretically, you can get up to 10400 / ((12+12)*8) = 54 samples per second!
However, there are two things that must be considered.
First, you're not supposed to monopolize the bus, because it is used for communication between the ECUs. In practice, you won't be able to get 54Hz even with the fastest microcontroller, because diagnostic messages have the lowest priority, and will be blocked out by messages with higher priorities.
Second, SAE J1978 requires the scan tool to wait 300ms between sending requests, to (a) allow more important communication to take place, and (b) allow the ECU to have enough time to process the information. This requirement effectively limits the refresh rate to 3 samples/second.
So ELM32x-based scan tools working at 2.4Hz or 4Hz already approach, or even surpass the maximum allowed refresh rate. Higher data rates are possible, but are not exactly legal.