IF: How long can a short visit (for a field trip, etc.) be?

In European Fellowships the researcher can certainly go to conferences that take place in a Third Country. With regard to short visits, these are permitted as well, as long as the concept of short visit is kept. We would like to refrain from defining a time frame of 1-2 weeks because we want to avoid the next question “what about 2 weeks and 1 day - visits”.
The key point is that the short visits should clearly not be secondments. Secondments have a clear impact on the project, are planned before, and have a particular scope – for example, without the secondment the final results of the project would not be possible. A short visit on the other hand will have a limited impact and could be something that arises during the fellowship, so mentioning it in the proposal is not always necessary.
We want to avoid that the researcher has spent 2 weeks in Canada for example, then the results are not ok so s/he has to go back for another 3 weeks, then for another 3 weeks, and like this slowly it becomes a clear secondment spent in a TC. Secondments in TC are not eligible activities and therefore will not be funded.
(Answer provided by REA)