I'm using tetracycline 250 for get rid of face pimples, how long do i have to take this medicine to get best results?

Here's the key: Don't ever use an antibiotic alone to manage acne. You also need to be using topical benzoyl peroxide 5% 2x/day or as you can tolerate. Avoid taking tetracycline with milk, antacids or iron. Expect a good result in 4-8 weeks. If it's not satisfactory, or you risk getting scars, consider a course of isotretinoin. It's not for the faint-hearted but it works.