I can't sit at my desk for more than 20 minutes. That's how much I hate studying.
この場合、次のような言い方ができますよ。
ーI can't sit at my desk for more than 20 minutes. That's how much I hate studying.
「私は机に20分以上座っていられない。そのくらい勉強が嫌いだ」
to sit at one's desk で「机に座る・机に向かう」
ご参考まで!