i have recently been pondering this topic for a while.. ( i have my own opinion on when its time for moving on.. simply when the environment becomes counter productive for my professional and technical growth)
this leaves for a rather broad reasons.. so i decided to get them out for discussion.
1. none or lack of growth within department or company..
2. PPM.. u can guess this or ask me for answer

Piss Poor Management
3. lack of skill development ( training) within job/ or developing of skills that will never be used at job
4. Pay .. theres nothing else to be said about this
5. hours no longer fit with personal/ family life ( this is if work is causing conflict at home)
hmm.. i think this is majorly it