Child Student Visa 還未批

兒子訂了20/8 機票到英國，15/7到北角打指膜，23/7收到uk decision making centre email 補交 letter of permission confirming use of money, relationship and permission to study in the UK. 直至4/8還未收到通知visa approved.

請問各位通常Child Student Visa要等多久才批？