Maybe there's an official answer, but unofficially I would have
thought that rainfall duration is difficult to define in any exact
terms for an AWS. Presumably you have to specify some minimum interval
between tips which you would interpret as one period of rain having
ended and before the next is deemed to have started.

If you have rainfall at eg 3mm/hr which is still pretty steady rain,
this corresponds to no more than 0.05mm/min or in other words 4
minutes between tips of a 0.2mm gauge. And it wouldn't perhaps be
uncommon to have a prolonged period of lighter drizzle at eg 1mm/hr,
which would cause a tip no more than every 12-15mins. So you might
want to set an interval of at least one hour between tips to denote
the boundary between periods of rain and even this would be inadequate
in some situations.

JGD
www.weatherstations.co.uk