How long must you keep someones personal belongings after they move out and say they will be back to get it and now its a month later?

Ask yourself this. If the roles were reversed and they were your belongings how long would you expect them to be kept and how would you feel if you came back and found that they had been disposed of.....

What you should do is contact the person by writing to them (if you can not do this directly then do so through a mutual friend) and tell them that you want the space and that if they do not come and collect their things (say in another month) then, as you need the space you will be throwing them out. This warns them of you clear intent and gives them the opportunity to collect before you take action (keep a copy of the letter).