Determining a rock age using volcanic ash layer
The main interest of tephrochronology (i.e. dating sediments using volcanic ash layer) is specifically its instantaneity (relatively to geological timescale of course). It is precisely used for higher-resolution dating. An example would be the Kawakawa/Oruanui tephra from New Zealand which is a good isochronous marker bed at 26.5 ka, spread over 1500km, but represents probably only a few months of volcanic activity (see e. g. Manville &amp; Wilson 2006)

Important factors vis-à-vis the usefulness of a specific ash layer however are its geographical extent (a volcanic event that will spread an ash layer over a whole basin would be more useful in that it will be used to correlate a large number of sites together) and maybe its volume (you need a minimum amount of material to work with, I'll assume).

Reference: Manville V. &amp; Wilson C. J. N., 2006. The 26.5 ka Oruanui eruption, New Zealand: A review of the roles of volcanism and climate in the post‐eruptive sedimentary response. New Zealand Journal of Geology and Geophysics, 47: 525-547.