App that blocks facebook use for maximum ~5 minut per day. Anyone ?

I have download k9 webblocker and blocked myself all kind of porn and sites that might show porn-realted images and posts.
I cannot block myself facebook completelly because I need it to be in touch with my University collegues.

However, I would be really thankful if someone would share with me any info on an app for pc and/or iPhone that blocks the use for facebook after for example 5 minutes of using it.
Anyone ?