While I have my own personal preference (which is to spell all numbers out until it gets too complicated, such as in decimals and ratios), it really depends on the context.
I would write it thusly:
The greatest loss in accuracy for the method is between a rate of one and two hours where, on average, the accuracy of the method dropped from two and a half to seven days.
Were your numbers higher, though, I would write it thusly:
The greatest loss in accuracy for the method is between a rate of 14 and 15 hours where, on average, the accuracy of the method dropped from 12.5 to 17 days.
However, you may (as I once was) be writing for a company or within an institution with an internal style guide that states that all numbers from one to ten should be written in full and anything above that should be written in numeric form, regardless of the context.
Context has a lot to do with what you should be doing. Mathematics, science and economics documents, which would all be number-heavy, would probably prefer you to use all numbers (though I am no authority there, so you should check with your individual institution), but were you to write this sentence or something similar in plain prose, then most institutions would prefer it to be written out as in the first example above.