We most of the time abbreviate it h (heures) and mn (minutes). But I assume we would understand hrs and mins with no ambiguity.

Here is a nice website that I just found for abbreviations, if you have some more questions.

About the use of it, I would say it is quitte common to abbreviate these words, as for any frequently used measure (s for second, km for kilometer…).

French follows the international standards and conventions and uses the official symbols¹ which are:

• a single h for hour

• min for the minute.

The only official SI time unit is the second. The minute and the hour are mentioned by the standard and their usage is accepted but they are not officially defined in the SI.

By typographic rules, an unbreakable space character is mandatory before all symbols² except °, ', and ".

11:39PM is 23 h 39 min, you might drop min here and use 23 h 39. If the context is clear, you might drop the h too and use a colon without surrounding spaces instead: 23:39.

Some people still use the legacy convention mn instead of min but it has been obsoleted in 1975 when mn was replaced by min as the official symbol for minute and m curiously accepted as a shorter form (see Journal officiel du 23 déc. 1975). The m form that collided with mètre was abandoned in 2012.

It took around 30 years for the usage of min to overcome the technically illegal mn :

Note that very few abbreviations and no symbols at all do take the plural in French so mins is incorrect and hrs is twice more.

¹ _{They are mathematical symbols, independent of the language used, not abbreviations. As such they never have an ending dot like abbreviations built by truncation.}

² _{To save space, this rule is sometimes broken and no space is used but it is nevertheless considered an error.}

The usual abbreviations for hours, minutes and seconds in French (heures, minutes, secondes) are the SI ones: h, min and s. These are also the abbreviations mandated by the official Québec rules. For minutes, mn is also commonly used, but less than it used to be. m is also common when used in combination with hours.

Putting an s at the end of an abbreviation to denote a plural can happen, but it’s less common in French than in English.

People would probably understand the English abbreviations in context, but French people would not write hr. The official Québec rules explicitly recommend not to use hres, which is not something I’ve seen in France.

Thus a time interval of 2½ hours would typically be written “2 h 30 min” (spacing optional). When there are hours and minutes but no seconds, it’s common to omit the unit for minutes: “2h30”.

For short intervals with minutes and seconds but no hours, you’ll sometimes find something like 4’33” but this is frowned upon (’ and ” are symbols for arc minutes and arc seconds, not for units of time).

We often use this to write the time as well, but it’s also common to use a colon between the hours and the minutes, e.g. 2.30pm is “14h30” or “14:30”. (Note that in France, the time of day is always written using a 24-hour clock since the beginning of the 20th century.)