How long has it been on? What condition does it look in - pristine, frayed, hanging by a thread? From new, I'd say not before 3-5000 miles, depending on type of use. Why are you worrying about such a cheap spare? Carry one in the boot, tightwad.
the normal interval for timing belt renewal is 80,000 miles ,but if you do a lot of stop start driving (short journey's ) school runs and so on then it is recommened that the belt be changed every 40,000 , but if there is signs of wear then i would get it changed a,s,p