For major defects such as those causing a danger to inhabitants or construction not meeting NHBC standards then immediately and urgently. Any issues of this kind should be resolved before moving in. For snags that are less serious this would depend on the nature of the snag. Some issues such as damage to floor tiles or scratches to windowpanes could involve a lead time while the developer orders a replacement, the wait could be weeks. On the other hand issues with cosmetic finishes could be addressed the next day. This of course all depends on how helpful the developers are. Some property developers employ staff outside of their construction teams to fix snags.