Here is the AppleScript code, which you can put in your workflow, by implementing an AppleScript item, and copy-paste the code in. The time is configurable, but you need an exact sleep time, which I can add if you could elaborate some more on the time. Here is the code via CulturedCode.
global quit_after, check_every
set quit_after to 2700
set check_every to 10
set minute to quit_after / 60
display dialog "Check is performed every " & check_every & " seconds. Things will be quit after " & minute & " minutesof system inactivity."
on reopen
display dialog "Check is performed every " & check_every & " seconds. Things will be quit after " & minute & " minutes of system inactivity."
end reopen
on idle
set idletime to do shell script "echo $((`ioreg -c IOHIDSystem | sed -e '/HIDIdleTime/ !{ d' -e 't' -e '}' -e 's/.* = //g' -e 'q'` / 1000000000))"
if (idletime as integer) > quit_after then
tell application "System Events"
if ((name of processes) contains "Safari") then
tell application "Safari" to quit
end if
end tell
end if
return check_every
end idle