It's a bit of a how long is a piece of string @Pauline32 as it will very much depend on your circumstances and what other sources of income you have.
Understandably mortgage companies are unlikely to take Airbnb income into account as it is not stable, guaranteed income.
When I bought a place recently, which I use for Airbnb, I based the mortgage on my employment income.
I used a mortgage broker who was really helpful. It would probably be worth seeing if you know someone who could recommend a local broker.