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There is no prescribed limit (except for mechanically propelled vehicles). Which appears to be the point that TheCatMan and I are both making, albeit in different ways.

It's an offence to be incapable, but, as yet, there is not test for what constitutes incapable. You may not, for example, be compelled to provide a breath / urine test, and it is not an offence to refuse one.

I guess it would be similar (or the same as) the roadside impairment test they use for drugs; walking in a straightline, finger to nose (your own, not theirs) blah blah. I doubt they would be arsed in all honesty.