Where can I find out when I installed an app? Is there a Play Store log file? 

Or should I be looking where apps are installed on the phone for time/date stamps?
Many apps available from Play Store show the install and update time of the installed apps. I've tested three of them and have given instructions to use them appropriately.
AM (Pre-release)
Install it → tap on three dots line in upper-right area → Sort → Last update.
If you click on an app entry there you can also see its Installer name in the App Info (scroll down to the bottom) section.
Applications Info
Install and launch the app → tap your app's entry.
Solid Explorer
Install and launch the app → from the left sidebar select Applications → User apps → tap on your app's entry.
(Click an image to enlarge it; order of images: Elixir 2, Applications Info, and Solid Explorer)




Command-line
If you want a command-line way, follow these instructions:
Setup adb in PC and execute the command:
adb shell dumpsys package

You might end up with overwhelming information about all the installed packages including the  date/time stamp you're seeking. If you want the said stamps for a specific package whose package name you're aware of, then enter:
adb shell 'dumpsys package PKG_NAME | grep -e &quot;firstInstallTime&quot; -e &quot;lastUpdateTime&quot;'     # replace PKG_NAME with the package name of your app

Its output possibly would be like,
firstInstallTime=2015-07-04 15:49:50
lastUpdateTime=2015-07-04 15:49:50
Note that the info is originally available in the file /data/system/packages.xml.
Alternatively, if you've Google Play Store installed, then take its backup using ADB, extract the backup to locate the database localappstate.db. Open that file in PC using an SQLite database viewer like DB Browser for SQLite or on Android using apps like SQLite Editor / aSQLiteManager. The said apps may need root access so I recommend to stick with the SQLite viewer for PC.
The table appstate in that file has the columns first_download_ms delivery_data_timestamp_ms. The latter shows when the app was installed and the former shows when the download was initiated by Play Store.
Limitation: Play Store wouldn't show date/time stamp for side loaded apps.