v10.3 allows 100K bars on demo accounts and 200K bars on real accounts.
It depends on your TF (and on the tick-by-tick option, for higher TF’s), a 4-hour TF has about 4 years of data history, while the 5-minute TF about 14 minths, 1-minute abour 3.5 months, 10-second about 5 days.
v11 (premium), still not supported by IG, allows 1M bars and even more splitting your backtest in chunks of 1M each (up to 4 1M chunks, if I remember correctly) on a PRT account; when IG start supporting v11 we’ll see whether they will support ALL those new features or just a subset of them.