I would say a task is complete when you have the villager selected and it says that their action is "nothing"... unless you just want them to gain experience, I would let them alone, unless they wander off the job, because even if it says "experienced gained", you won't get the percentage completed up any (in the case of building, or the tech points (in the case of researching). The one thing that does help you to not only gain XP, but also get the task completed faster is picking up the villager after they put the food in the bin and dropping them on the berry bush (or farm land) - and the breeding trick.