The default timeout is 10 minutes, so the iterators in the InputFormat's RecordReaders would have to exceed this amount when returning a single record (progress is reported implicitly when a mapper receives a record). I suppose AccumuloRowInputFormat is more of a risk if a row has lots of cells, but 10 min still seems like plenty of time.
Since waiting more than 10 minutes for a single record is somewhat abnormal, users that anticipate this behavior could set the timeout threshold by adjusting the 'mapreduce.task.timeout' variable in their Job. Set it to 0 and the task won't timeout.
Is there a scenario that I'm missing? If not, I think this ticket could be closed out – perhaps just mentioning the timeout var in the javadocs.