Hi Kath. A healthy adult needs 1000iu daily. However if you work indoors like I do for daylight hours and use sunscreen(sensibly), then the latest recomendations are 2000iu daily for the winter/ Autumn months, 1000iu Spring/ Summer. When you have low levels it can take months for them to climb. The only way to know if you need to continue such high dose age is a regular blood test. Don't drop it until you have new bloods. Your GP will know.
Incase you are wondering 30 years+ pharmacy