Backtesting from Two Years Back

Hi,

It seems like the tick datum is working from 6 months to present. What I would love to do, is to backtest my strategy from the last 2 years. I'm not really sure how far back in time I can go. I don't get a warning when I try to retrieve data from too far back. Either it will finish quickly (not grabbing the data) or when I try to run a backtest, it says not enough data when attempting.

I tried Protrader, LMAX, FXCM, and Oanda. I can't seem to find a reliable database for retrieving data. I guess my question is this: How far back can I test my strategy on? My guess is 8 months, but I got lucky once and was able to get it to work for a year range.

Thanks