If you can, how long would it take to double my memory capacity?

Yes, memory has a muscle but it also has to be assisted w/ planning and paper (notes) reminders. Especially if you have a lot to do daily. Memory development comes w/ practice. Make mental notes to yourself and practice your recall. When taking test or studying make a game out of the curriculum. Most important, no one knows themselves better you. Practice what helps you best and I am sure you will develop better memory over time.