This varies greatly from startup to startup, but if you are not a founder it will be very very small. Early in my career I was a very early employee (think sub 10 employees) of a startup and was compensated with about 0.2% of the company. Later in my career I joined a very late stage startup (think sub 500 employees) and was compensated with about 0.007% of the company.
Also note, that when I say compensated, that was with a standard 4 year vesting agreement.
Also note, that when I say compensated, that was with a standard 4 year vesting agreement.