Doctor told me my stomach isn't emptying as if should. He gave me Nexium and reglan (metoclopramide). When will I start to feel better??

Depends. Were you diagnosed with "delayed gastric emptying" or gastric outlet syndrome? The medications should be reviewed with your physician and they can take 2-4 weeks to become effective.
Soon. You should begin to feel better within a few days of starting treatment. If you don't, you may need further evaluation.