For platies with ich you should (assuming platies is all that is in the tank) raise the temperature slowly to 83 degrees farenheit and do increased waterchanges, holding that temperature for 14 days (2 weeks). Slowly raising the temp means a couple of degrees an hour. Increased waterchanges can be anything from twice your normal rate and up, because even daily waterchanges wouldn't hurt, but aren't usually necessary.
Platies should be kept normally at 78-80 degrees farenheit, but the increased temperature for a fortnight will kill the ich parasites. Be sure to gradually lower the temperature in the same way you raised it, if you choose to do the easy, chemical-free, treatment for Ich.
