E.g. Wikipedia gives as example:
Levada Center, Russia's only independent polling agency, received between 3 and 1.5% of its total budget from abroad. It was issued with a public warning that it would be eligible under the law. Levada said it suspended foreign funding in 2013. In 2016, the polling agency was named a foreign agent, barring it from work on the upcoming election. Levada's director stated that the designation may mean that Levada would be unable to continue its work as a pollster.
So even 1.5% foreign funding is enough to qualify as foreign interference, and thus a foreign agent under Russian law or regulations? It's unclear from Wikipedia if there is an official threshold.