"i want to know, how many second AT89C51 take one bit data from I/O port ( p0.0). "

A very ambiguous query. Unless you mention what crystal ( or in other words what speed ) you are using I dont think your query can be answered.

Even assuming you mention it, I dont know what you really want.

LOOP : MOV C, P0.0
       JMP LOOP

The above could be the fastest. But are you happy with that? ( BTW it is a useless set of code )

Raghu