This has been a long due request. So Before May 22 PowerApps Notify() function has a default timeout of 30 seconds. Which was en eternity for some of the people.
Microsoft finally listened and they have updated the Notify() function.
Here are the changes
The default timeout (i.e. if you do not provide a timeout value) is 10 seconds.
The timeout value could be between 1 millisecond to infinity
The Timeout value needs to be entered in Milliseconds
If you want an infinite timeout you need to enter 0 as a value.
Read the official docs here
Fun Trivia: Notify function was originally called ShowError and was focused on showing errors and then it evolved to more general Toast Notification feature.