Is doubt concerning ablution after finishing it not to be taken into consideration, and does this ruling apply immediately after finishing or after a long time, and what is its legal evidence?

Doubt after completing an act of worship, be it purification, prayer, or otherwise, does not affect it, regardless of whether a long or short time has passed. This is because the default assumption for the actions of a mukallaf (a legally responsible individual) is that they are performed completely, and because entertaining doubt after performance leads to hardship and difficulty, which are alleviated in Islamic law, as Allah Almighty says: "And He has not placed upon you in religion any difficulty," and His saying: "Allah intends for you ease and does not intend for you hardship."