More generally, doing personal things like phone calls, emails, etc. while on company time. How far does the "everybody does it" rationale go? I believe there is some category in halakha that accounts for reasonable assumptions.
It depends on your boss and on local custom.
If local custom generally accepts short personal business during work hours, and your boss never stipulated otherwise when you were hired, then it's OK. If local custom can't be determined, then you need to honestly assess whether your boss would mind.
A posek once provided a rule of thumb: if the boss would walk by, would you hide whatever it is you're doing? If so, it's probably improper.
I'm not sure if this is an answer or a comment, since I don't actually know R' Yitzchok of Vorki's source.
In a story published in Parshat Yitro of Shlomo Yosef Zevin's book, translated by Artscroll as "A Treasury of Chassidic Tales", a story that addresses this is brought.
In short, R' Yitzchok of Vorki (before he became Rebbe) once said that "when one is not actually needed on duty, one is allowed to steal a little time for the study of torah". You can read the full story, "An Uncommon Thief", here.
To fill in on Adam's answer:
Rambam concludes his Laws of Hiring with the following:
ט אבל אין הפועל רשאי לעשות מלאכתו בלילה, ולהשכיר עצמו ביום, או לדוש בפרתו ערבית, ולהשכירה שחרית; ולא יהיה מרעיב ומסגף עצמו, ומאכיל מזונותיו לבניו, מפני גזל מלאכתו של בעל הבית, שהרי יכשל כוחו ותחלש דעתו ולא יעשה מלאכה בכוח.
י [ז] כדרך שמוזהר בעל הבית שלא יגזול שכר עני, ולא יעכבנו--כך העני מוזהר שלא יגזול מלאכת בעל הבית, וייבטל מעט בכאן ומעט בכאן ומוציא כל היום במארה, אלא חייב לדקדק על עצמו בזמן מלאכה, שהרי הקפידו חכמים על ברכה רביעית של ברכת המזון שלא יברך אותה.
יא וכן חייב לעבוד בכל כוחו, שהרי יעקוב הצדיק אמר "כי, בכל כוחי, עבדתי, את אביכן" (בראשית לא,ו). לפיכך נטל שכר זאת אף בעולם הזה, שנאמר "ויפרוץ האיש, מאוד מאוד" (בראשית ל,מג).
A worker is not entitled to work at night, then hire his services out at day; nor to use his cow to thresh at night, then rent it out the next. Nor can he starve and afflict himself or give the food he should eat to his children -- all this is stealing from his employer, as he is weakening himself mentally and physically and will be unable to work with full strength.
Just as the employer is warned against stealing, delaying, or withholding his poor worker's wages, so too a hired worker can't steal work from his employer -- wasting a bit of time here and a bit of time there, until the whole day has gone down the drain. Rather, he must be exacting upon himself when working, as the Sages even let someone skip the fourth paragraph of Grace After Meals [for hourly workers who had to get back on the job].
Similarly he must work with all his strength, as Jacob stated ... and he was rewarded accordingly.
This discusses the hourly worker doing physical labor. (I believe most moving services today fall into this category.)
The first disclaimer is that halacha treats any work agreement between adults as subject to the terms of hire, or if unstated, the standard norms at that time and place. As Barry said above.
כשם שמתנה אדם כל תנאי שירצה במקח וממכר--כך מתנה בשכירות, ...
Just as any stipulation can be stated with regards to a purchase, so too with a rental.
השוכר את הפועלים, ואמר להם להשכים ולהעריב--מקום שנהגו שלא להשכים ולא להעריב, אינו יכול לכופן. מקום שנהגו לזון, יזון; לספק בתמרים או בגרוגרות וכיוצא בהן לפועלים, יספק. הכול כמנהג המדינה.
If one hires workers [without stating special terms], then tells them to show up extra early or stay extra late -- if this is a place where the practice is not to do so, he can't force them. If it's a place where the practice is to feed them, he must feed them; to supply dates, dried figs, or the like, he must supply. All follows the practice of the land.
The second disclaimer is based on Rambam's opening line -- you're helping no one if you're destroying yourself and doing a substandard job. It could be argued that for the long-term employee in an information-based occupation, some web surfing can "refresh tired workers and enhance their productivity."
Again, it's up to your employer, as Barry said.
In a recent article, Rabbi Meir Orlian writes:
The details of [employees'] work obligation [described in Choshen Mishpat 337:19-20] depend on what is customary in that time and place (Choshen Mishpat 331:1). If it is customary to allow workers a short call home or to the spouse during the course of the day, it is permitted. If it is customary to allow time for the workers to go to Minchah, it is permitted. Similarly, if it is customary in that profession to string together a number of part-time jobs, that is acceptable. However, the employee should be careful not to overextend this allowance and make numerous calls, spend time to handle personal needs during work hours, or spread himself thinly so that he cannot properly fulfill his responsibilities.
This article is intended for learning purposes and not to be relied upon halacha l’maaseh.
The RaMBa"M actually deals with this in Hilchot Sechirut, in the very last halakhah of the final pereq. He even quotes this week's parashah (b'chol kochi avad'ti et avichen)
וכן חייב לעבוד בכל כוחו, שהרי יעקוב הצדיק אמר "כי, בכל כוחי, עבדתי, את אביכן" (בראשית לא,ו). לפיכך נטל שכר זאת אף בעולם הזה, שנאמר "ויפרוץ האיש, מאוד מאוד" (בראשית ל,מג).
I heard here from this Rav and I heard it hinted by a Rav Mutzafi Shelit"a and they say it Asur without permission.