How often should a piano be tuned?

Depends on how much playing it has had, minimum once per year.

How long does it take?

About an hour and ten minutes.

How much does it cost?

Current rate is £45 but all tuners are reviewing charges due to increased fuel costs.

I called a tuner and he/she said the piano was unable to be tuned!

You have an honest piano tuner who rather than having a bad day was politely telling you your piano was past it! You should incur a small call out charge say about £20 for their time and advice.

What does slack pins mean?

You have an old piano, which has dried out and the tuning pins are loose in the tuning plank.

I’m kind of scared about who I engage to tune my piano.

You are right to feel this way. Unfortunately piano tuning is not regulated like corgi gas etc. and there is little trade representation and many “cowboys” abound….best go on recommendation.

I bought an old piano at an auction for my son/daughter to learn piano

Worst thing you could have done.. Unless your very lucky. If you were learning to drive a car would you put your son/daughter in an old banger? They will have no chance.