All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
A woman who goes out of her house without wearing the Hijab, does not nullify her ablution whether she is outside for a long or a short period. However, she should fear Allaah and not go out of her home without Hijab. If she goes out without Hijab and marriageable men see her, she is sinful.
For more benefit and details, please refer to Fataawa: 83033 and 83702 about the evidence of an Islamic Hijab and its description, and Fatwa: 84093 about the nullifiers of ablution.
Allaah knows best.