How long does it take for triglycerides to go down after taking fish oil tablets?

No promises: I doubt that they'll go down from taking those pills, period. Decisions about managing elevated triglycerides, especially if that's your only finding (and whether you should address them at all), should be made with your physician. Why not get yourself back into a fitness-focused lifestyle like when you were a teen? I'll bet you'll have a lot more fun and see those triglycerides plummet.