We use for and since to say how long:
• Jaya is in Mumbai. She has been there for three days or since Monday
Ramesh has been in Jaipur for six months.
• We've been waiting for two hours.
• I've lived in Mumbai for a long time.
We use for + a period of time (three days / two years etc.):
We use since + the start of the period (Monday / 9 o'clock etc.):
We use ago with the past (started/did/had/was etc.).
• Suman started her new job three weeks ago.
• 'When did Tanmay go out?' 'Ten minutes ago.'
• I had dinner an hour ago.
• Life was very different a hundred years ago.