The most basic limitation on timescale is the light-crossing time. The term includes both quasars and BL Lac objects. The vast assortment of observations of spectral variability of blazars have revealed a diverse range in characteristic behavior. There are many similarities and differences both among sources and among flares within the same source. At the risk of oversimplifying, I list the major properties of spectral variability that I feel best exemplify the nature of the phenomenon, while recognizing that there are exceptions to these patterns.
Flares usually begin at high frequencies, then propagate to lower frequencies.
Some flares, however, develop simultaneously over a wide range of frequencies before propagating to lower frequencies, and still others do not seem to move to lower frequencies at all. To first order, the multi-waveband spectra connect smoothly from the radio to the infrared. The timescale of variability is shorter at the higher frequencies. The polarization behavior also tends to be much more erratic at these frequencies. The flickering occurs on the same timescales as found in the optical.