How long have you been a .......

Hi guys,
I was wondering how one would ask. How long have you been a .........
I know that using dono gurai or Dono kurai you can ask questions like: dono gurai jikan ni kakarimasu ka to ask how many hours did it take etc.

However when it comes to nouns like teacher or doctor I'm not sure the grammar structure. Would you need to use naru.

Example how long have you been a doctor?

Ishi wo nuru koto ga dono gurai desu ka

Or would you stipulate time since?

Ishi wo naru kara dono gurai nan nen desu ka
Thanks

Thanks.

医師になってどのぐらいになりますか？
Ishi ni natte donogurai ni narimasuka?

なる is a intransitive verb so cannot be used with を.

It's 時間がかかる. Jikan ga kakaru.

Alternatively, どれぐらい医者をやっていますか (dore gurai isha o yatteimasu ka).