how long does it take to go bad? have you done anything else to the engine since before the starters decided to go out?

Do you replace the whole starter assembly? If you do there and it only clicks and then hesitates to roll over you should check the starter CONTACTS. Those are the general fault and if the starter contacts have been replaced then you should check to make sure that they are installed LEVEL in the starter solenoid area. This is generally the only thing that wears out in Toyota brand starters and some lines do not stock nor recommend replacement of the contacts for the level purpose because one bad install can cause everything to go.