On which type?
What is your current experience (hours and types)?
Are you talking about small operators (eg. single pilot ops) that call themselves airlines, or are you referring to operators of multi-crew aircraft?
Do you already have a command rating on the type of aircraft operated by the company you had in mind?
For the mainstream, multi-crew operators, it might be as short as 6 months - and it could also be 12 years +
How long is a piece of string?
(A typical Aussie
)