How much gearbox oil is too low?

[komichal]

When the shop changed my left axle recently, they did not change the axle sealing properly and the gearbox oil was leaking for a month.
When I discovered that I immediately topped the oil up - it consumed almost a litre of oil.
I was driving the car daily and have not noticed any issue during that period.

So I would probably recommend to wait till your oil comes in but if you are in a desperate need of driving (need to take someone to a doctor etc.) then I would not worry about it and just drive...