We're only supposed to work until 5:00 p.m., but I often work overtime until 8:00 p.m.
We're only scheduled to work until 5:00 p.m., but it's not unusual for me to work an extra three hours overtime.
この場合、次のような言い方ができますよ。
ーWe're only supposed to work until 5:00 p.m., but I often work overtime until 8:00 p.m.
「本当は仕事は５時までだけど、よく残業で８時まで働きます」
to work overtime で「残業をする」
ーWe're only scheduled to work until 5:00 p.m., but it's not unusual for me to work an extra three hours overtime.
「仕事は５時までの予定だけど、３時間残業するのは珍しくない」
not unusual で「珍しくない」
to work an extra three hours overtime で「３時間残業する」
ご参考まで！
I often have days where I work overtime until 20:00.
「20:00まで残業する日がよくあります。」は英語で「I often have days where I work overtime until 20:00.」と言います。また海外では24時間の表記が苦手な人が多少いますので「8PM」を使っても問題ありません。
お仕事は17:00に終わりますが、20:00まで残業する日がよくあります。
My work ends at 5PM, but I often have days where I work overtime until 8PM.
カナダ 
