Can I search games by estimated play time?
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:
https://store.steampowered.com/tags/en/Short/
Also on SteamDB, you can filter games on 'Short' tag (which seem to come from tags set by users on Steam Store):
https://steamdb.info/tags/?tagid=4234
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.
I am actually working on a website with a focus on estimated playtimes of steam games.
You can check it out at: https://howlongis.io
The estimated playtimes are based on data collected from the Steam API. The main reason why I stopped using HLTB and started my own website:
Objective data: I wanted to use objective data from steam vs subjective data inputted by users on HLTB (there is a steam integration on HLTB so in the best case scenario the data reliability is the same).
Data distribution: I wanted to represent data in different ways for example showing a distribution with percentiles where casual players can find themselves around the 25th or 50th percentile and completionists or real fans can look at the 90th percentile to get an accurate idea of how long they can expect to play the game. I have found this to be incredibly useful.
More data: HLTB has limited data for games of smaller developers and lesser known games. If you like indies, you will probably know that HLTB sometimes only has a handful of datapoints for these games which is statistically not very relevant. Using 45 million+ datapoints from the steam API, I usually have more available data to determine playtimes.
Improved search: This is not live yet but I'm working on an improved search functionality to help with the issue posted here. I want to allow people to look for games of a genre or steam tag with delimiters for the playtime and review score. Note that this is not available yet but soon will be.