Cable Length

The maximum cable length is about 6,371 km. Any longer than that and you will be wasting cable. Now depending on your communication technique, acceptable noise level, signal level, bandwidth and a blur of other unstated criteria you can get your signal from one point to another with less cabling.

is that a standard length in which standard ?

any parameters can aafect this length

is that a standard length in which standard ?

Have a look at the link in post #2. The value is not a practical cable length for foundation fieldbus.

any parameters can aafect this length

Where you measure the circumference (the earth is not a perfectly round sphere).

Half the distance from the middle to either of the cables two ends.

For foundation fieldbus a quick internet search will give you the answer (there is no single answer that suits all applications).

FF HSE or H1?
FF HSE is ethernet based and has the limitations of ethernet: 100m over copper, longer over FO.

FF H1 limitations:
- Without repeaters, the maximum length of an H1 segment can be as long
as 1900 meters. By using up to four repeaters, a maximum of 5*1900 m =
9500 m can be jumpered. The short spurs from the field device to the bus
are included in this total length calculation.

- The maximum length of a spur is limited to 120 meters and depends on the
number of spurs used as well as the number of devices per spur.

Juxt to elaborate further on FF , high speed Ethernet following are cable limitations :

1. On twisted pair - at 100Mbps - 100 mtrs

2. On fiber - at 100Mbps - 2000mtrs.