In the Steam store, is there any way to search for "short games", e.g., games with an estimated completion time of less than 1 hour? I did not find such option in the advanced search link.
As DrFish mentioned, this is a feature that currently does not exist within Steam. However, if you're willing to use external sources, I recommend taking a look at HowLongToBeat.com.
HowLongToBeat is a community driven site where players input their stats on how long it takes them to beat the story, side content, etc... across multiple platforms. The searching options allows you to filter down games by PC and the average number of hours it takes to play through the game.
Since it is community driven and outside of Steam, it may not have every game posted, but it's useful nonetheless.
As Parrotmaster pointed out, it is good to take a note of the release date and number of reports given. When games are first released, it may take a couple of days or weeks to get accurate statistics due to players rushing through the game. Those who rush through the game tend to skew the results, making it seem like games are shorter than they really are.
The info is not yet available in that form on Steam, but you can filter by tags, curated by the community, so you can assume it is subjective to a degree:
Also on SteamDB, you can filter games on 'Short' tag (which seem to come from tags set by users on Steam Store):
Which will yield all games tagged yet as such.
Whether the two sources yield the same results is unclear to me: SteamDB delivered 1,122 hits as opposed to Steam's 807 results. This is with a default user, that is, without filtered out hits based on preference.
Combing both answers by Wondercricket and DrFish:
I use Depressurizer to automatically group my Steam library by tag, and more...
In a recent update they added HLTB integration. To quote from their readme:
HLTB: This lets you categorize games based on the times from http://howlongtobeat.com/ which indicate how long it takes to beat a game. To use this method, you must create a set of categories to assign, and define the criteria for a game to be placed in that category. Each game will only be assigned to the category for the first rule that matches it. If a game does not match any rules, it will not be assigned to a category.
Using the auto-category function of Depressurizer, I have groups prefixed with "(HLTB)" alongside my "(TAG)", "(YEAR)", and "(SCORE)" prefixes.