How fast can stimuli be administered for evoked potentials? I'm looking for a reference about the ability for sensory stimuli to generate separate evoked potentials in the brain. 

What is the maximum repetition rate for sensory evoked potentials?
For auditory brain stem responses (ABRs) the click rate can be really high, as the auditory system is exceptionally fast, in the order of tens of stimuli per second (e.g., Rodriguez et al (2010. This will still yield a good ABR. At around 40 ms, the transient ABR shifts toward a steady state response, in which peaks of the previous ABR start to overlap with the following (Korczak et al., 2010).  

The visual system, however, is a lot more sluggisch and needs more time to recover from prior stimulation. For example, VEPs are generally recorded at a few stimuli per second. The transition from a transient response to a steady state response occurs at around 6 - 10 Hz (Fishman et al (2001). 

References
- Fishman et al., Electrophysiologic Testing, 2nd ed. FAAO
- Korczak et al., J Am Acad Audiol (2012); 23: 146–70
- Rodriguez et al., Brazil J Otorhinol (2010); 76(1): 96-101