there will be no direct answer too this question ,.......some terriers get better with every dig, some slower , some wont , some will,.......some start early , some late,.....every dog is different...
every owner is different , some will wait , some wont,......
so every answer will be different........what I will say is after 18-24 months you will know if a terrier is going to be up too your own standard ,......and with experience comes the number of digs and the number of digs is a wiser dog ......
some young dogs will work like they been doing it for years , some wont......
so the number