the HK governmtent does not recognise common law marriage(that is living as husband and wife but without the legal commitment)...
if neither of you are adverse to marriage, go for it...
just remember, even if your partner has a dependent visa it does not mean that she can work. she would have to apply for a work visa just like everyone else. what the dependent visa will allow is that she can live in hk without having to exit every few months(this can get tricky unless she exits regularly for different lengths of time and to different destinations)
the government occasionally allows engaged couples to enter togetherif they can prove that they plan on marrying in the very near future...i haven't heard of anyone successfully going this route though... maybe someone else here has this experience?