As Dennis said, if you're using the built-in sweep function, you'll have to check the manual. See Section 10-16 (That's section 10 page 16, not sections 10 though 16) of the Keithley 2400 User Manual.
The Keithley 24xx series has a measurement speed in units called PLC (Power Line Cycles). The default speed is 1PLC, meaning a measurement is taken every 1 power line cycle, or 1/60th of a second (16.67ms). 24XX's can go from 0.01 PLC (every 0.16ms) to 10 PLC (every 166.6ms). The faster you measure, the lower precision you get.
To set this value programmatically, the command is
:SENSe:CURRent:NPLCycles <n>
:SENSe:VOLTage:NPLCycles <n>
depending on what you're sensing and where <n> is the number of PLCs from 0.01 to 10.
Another factor that will determine the time between data points is the SDM cycle. These are more complicated, look at your Keithley user manual for more information. Look at Section 6 and Section 11 for more info.
Note:
PLC times are based on a US cycle of 60Hz.