The number of mainland sex workers working in Hong Kong is difficult to estimate because the women come and go frequently, staying between two weeks to three months on two-way permits. An increasing number are on individual visit permits, which was introduced in July 2003, they usually stay for a week. It is illegal for mainland visitors (including sex workers) to work in Hong Kong. If caught, they are charged with breaching the conditions of stay, an act liable to a maximum penalty of two years' imprisonment and a fine of $50,000 (most are jailed for three months and then deported).