Would there be a slight propaganda / morale victory if Ukraine kept pounding one of the better Russian units until they break? The Russian bloggers already trumpet the units defending near Tokmak as heroes.
Also, how long would you need to continuously batter one of the better Russian units, well-entrenched, with somewhat disrupted supply until they break? (Longer, of course, if they are able to bring up replacements.)
I believe the 101st Airborne held out, surrounded in Bastogne during the Battle of the Bulge six days until relieved.