I stayed in Arusha, Tanzania for 1 month from the 3rd of June till the 2nd of July volunteering in an orphanage. I cannot recommend this trip enough! i think 1 month is the perfect time limit to go as you have enough time to really get to know the other volunteers and locals that work there. I also think that going in June/July time is perfect as there are always other volunteers around and the weather isn't too hot.
It's difficult to describe how it's so enjoyable but it is definitely worth doing through PMGY.