In most definitions of a supercell there's talk about a persisting rotating updraft.
For example in the Storm Spotters Glossary:
*Supercell - A thunderstorm with a persistent rotating updraft. Supercells are rare, but are responsible for a remarkably high percentage of severe weather events - especially tornadoes, extremely large hail and damaging straight-line winds. They frequently travel to the right of the main environmental winds (i.e., they are right movers). Radar characteristics often (but not always) include a hook or pendant, bounded weak echo region (BWER), V-notch, mesocyclone, and sometimes a TVS. Visual characteristics often include a rain-free base (with or without a wall cloud), tail cloud, flanking line, overshooting top, and back-sheared anvil, all of which normally are observed in or near the right rear or southwest part of the storm (Fig. 7). Storms exhibiting these characteristics often are called classic supercells; however HP storms (Fig. 3) and LP storms (Fig. 5) also are supercell varieties.
How long is persistent?
I guess that it must be long enough in comparison to the thunderstorm's life cycle to be persisent, to rule out short lived rotating (wannabee) wallclouds..... So, half an hour is persistent enough?
What do you understand under persistent?