how much to raise rent

[George Cocokios]

Have a rental property in Phoenix metro area.  Rents have risen quite a bit in the past 3 years and i have not increased their rent but am considering raising it when it comes up for renewal this spring.  How much should I raise the rent  ?  I'm currently about $120 less than the cheapest rental on the market today but I don't want to freak them out with a huge increase.  Any suggestions on what to do ?

George.