Have "Remember Me" checked by default on logins, configure how long a login is remembered, or disable the "Remember Me" feature al
By default, if you log in without "Remember Me" checked, WordPress keeps you logged in for up to 2 days. If you check "Remember Me", WordPress keeps you logged in for up to 14 days.
You can't (and probably shouldn't). With a session length of less than an hour you risk timing out users too quickly.