It is the female equivalent of male semen.
Does happen all the time or just during a certain time in a females cycle?
Well you sure should ask it and have every right to know and clear your doubts. As for your question, it is the lubrication that you are calling "white goo". When a female gets aroused sexually, her vagina releases this lubrication. When released in minute quantities, it seems transparent. But when highly aroused, it is released in a more denser quantity making it appear white in colour.
I actually think it's called discharge.
Its either from a yeast infection or you use a certain soap that leaves residue in your crevices
Its called vaginal discharge.