Well I suppose "reasonable period" is subjective, unfortunately. I hate that myself, because I don't like introducing subjectivity. However, my theory is that for COMPARISON purposes, you must ensure consistency.

For example,
I could look at a Performance Report and see that a bot is "good" or maybe "excellent". Those would be subjective evaluations. However, my theory is that I could not look at two bots and know -- in an objective way -- if one was better than the other... at least without having knowledge that they were developed with the same amount of history. One way I think you could answer that question would be knowing the relationship of the BarPeriod with the total amount of data.

Not knowing that relationship means you would also affect the statistical accuracy of the logic, I believe (note: I confess I am NOT a statistics or math expert!). The reason why is because of this example:
240minute barperiod strategy across 10 years data
is not comparable to 15minute barperiod strategy across 10 years

There would be 16x more opportunities for trades in the 15minute strategy across that dataset. Therefore, it is not a fair comparison with the 240minute strategy across the SAME dataset.

Now don't get me wrong... I'm not advocating necessarily that it's bad to get rid or avoid testing on max history available. I'm only suggesting that perhaps it is not best for COMPARISON purposes.

If you believe that max history is the best philosophy, then one way you could accomplish that is like this:
1) determine the History ratio of your longest BarPeriod strategy across the max history available
2) Adjust testing history on all other BarPeriods to match

This provides a more similar way to COMPARE different bots without introducing the unfair bias of additional trade opportunity history. You could also "maybe" argue that more history is less relevant and therefore potentially "bad".

Separately, in my design process, I'm comparing first with the limited/equalized dataset but also testing with max history. The max history test is just for my information, but may not be appropriate for comparison purposes.

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Btw, to answer your question -- I think I mean "reasonable period for optimization purposes" ... because in the stage where I would be limiting history, it is mostly for the purpose of finding the best trading logic.