A modification of the script at http://hints.macworld.com/article.php?story=20100801214648362 to prompt for a delay and minutes of control. If you want to set your own defaults, change the 180 or the 30 to whatever amount you want.

gistfile1.applescript

on run argv
  set defaultTime to 1
	
	try
		set myTime to item 1 of argv as number
		set myDelay to 0
	on error
		display dialog "Enter time to block in minutes" default answer 180
		set myTime to (text returned of result) as number
		display dialog "After how much time" default answer 30
		set myDelay to (text returned of result) as number
	end try
	
	
	
	tell application "Usable Keychain Scripting"
		tell current keychain
			set myPass to (password of first generic item whose name contains "SelfControl")
			-- eliminate invisible characters, or "gremlins," from password
			set x to quoted form of myPass
			set myPass to do shell script "echo " & x & " | perl -pe 's/[^[:print:]]//g'"
		end tell
	end tell
	
	delay (myDelay * 60)
	
	tell application "SelfControl" to activate
	
	tell application "System Events"
		tell process "SelfControl"
			tell slider of window "SelfControl" to set value to myTime
			click button "Start" of window "SelfControl"
		end tell
		tell window 1 of process "SecurityAgent"
			with timeout of 15 seconds
				repeat
					set tryAgain to false
					try
						set value of text field 2 of scroll area 1 of group 1 to myPass
					on error
						delay 1
						set tryAgain to true
					end try
					if not tryAgain then exit repeat
				end repeat
				click button 2 of group 2
			end timeout
		end tell
	end tell
end run