Hey Alan, not sure what you mean by accurate but I’m assuming you mean reliable and robust.
To be able to have any kind of faith in the strategy it’s important that it has been backtest over a significant period and has made a significant number of trades in that period.
Personally I use about 5 years data for both M15 and H1 and set the minimum trades to 300. Obviously this will be 4x more bars for M15 so will take longer to generate strategies, but it is also easier to pass the minimum 300 trade criteria for M15.
Hope that answers your question? Using less is fine as is using more, that’s just what I use ;-)
Also to get this amount of data use the Premium data or historical data app