In the category I have put in the publication days 0.(Indefinite time)
It is also possible with a plugin to have the user select different days.
In the Backoffice Manager Pro add-on it says:
The deadline will be extended by -0- days when the listing is republished.     (Leave blank or zero to get # days based on category expiration days.)

I put 0 or blank but the extension is always for 30 days and the default category time is not applied. How to fix it?

Its not possible.
It is also not going to work for you if listings are set not to expire