Depends on what makes you feel comfortable!
I guess you could look at your insurances (if you have 'ill-health' cover of some sort), what social security you might be able to get and at what point, and at what help parents or others may be able to provide for you if you really hit the bottom.
This might give you a better idea.
Also, depends on whether you mean 'salary' or actual 'expenses' - figure out exactly what you NEED (taking into account if you are laid up you may not need gas for the car, but you would probably be spending more at home on heating and electricity and so on).
Some would say 1 month, others 3 months and others 6 months.
Also, what is the lead time in your work between doing work and getting paid.
Say you are laid up for 2 months.
Then you do some work - how long would it be before you see your first invoice paid?
Personally, my view is get 1 month expenses down as quickly as possible and then build up to 6 months over time.