Mt5 will download the data when back testing and so you will be able to have a more than adequate amount of data. I would recommend back testing with full tick data base don real ticks and check that the % of real ticks quality is at least about 90%. If not then I would recommend importing quality tick data or back testing on a broker that has better quality data (Such as darwinex or dukascopy for example). For M5 and M15 5 years of data is ample in my opinion