At what point do lesbians count their virginities to be lost?

Out of curiosity I was just wondering how 'virginity' is measured in lesbian relationships. In a heterosexual relationship its clear as its through penetration of the penis but when you're a girl and you're having sex with a girl there is usually no penis involved so at what point would you no longer call yourself a virgin. Or are all lesbians virgin if they've never used a dildo? Just wondering any general or personal answers would help lol!