Some people masturbate in the shower, since that is the only place they had privacy growing up. Men sometimes soap up their penis and go to town. Of course, if you have never masturbated standing up, this may take some adjustment, but it does add to your options. This gives your roommate time to plan to be invisible, and you a block of time to do your thing, although Alice is not suggesting that you try this tactic every day. If you are worried about starting to masturbate and then hearing the key in the door, try a code, like placing something prearranged on the doorknob, i.e., a tie or a ribbon, to signal that you don't want to be disturbed.
Many men can, and do, come within two minutes of beginning to masturbate.
Similarly, Alfred Kinsey found that many women orgasmed through masturbation within five minutes, a brief amount of time. You do deserve at least one orgasm a day, and, if you're creative, Alice is sure you can come up with a way to find the privacy to give it to yourself.