How long is 25 hour half life?

Depends.: I am assuming you are talking about the half life of a drug. A drug with a 25 hours half life means that after 25 hours, the blood concentration of the drug will be halved. So at hour zero it's 100%. At 25 hours , the concentration is 50%. At 50 hours, it's 25%. At 75 hours, it's 12.5% etc. Please note that using half lives is an oversimplification and doesn't always correlate with drug effect.

It changes daily: A 25 hour half life means that in 25 hours half will be present and in 25 hour half of that will be present and so on