Is it okay to masturbate a lot while living with your girlfriend?

That's up to her: No two couples have the same expectations of one another. No two people have exactly the same sexual needs. If either of you thinks that the other will never think of anyone else, or never touch themselves, you are setting yourselves up for a certain disappointment. Learning what you can realistically expect from another person is a major step toward being a fulfilled adult. Best wishes 2u both.