how long rj 45 cable can use

100 meters is the MAXIMUM a cable can be.  Beyond that (and probably nearing it) you'll lose  signal strength and thus speed.  You can put a switch or hub in the middle of two 50 meter cables and get better results or if you have to have such long runs, try either Cat6 cable (may not help) or use a Fiber cable - much more expensive, but with distance limits FAR above that of copper CAT5/6 cables.  And of course you'll need the equipment to connect a fiber cable to.

yes 100 meters is theoretical maximum I'd suggest 75 meters is realistic maxumum
cat6 also is 100 meters

You may get  a slightly longer cable by using stp instead utp

but fibre or repeaters is the only real way to get long cable runs

u can test your cable using a LAN Tester and pinging from the server like this

ping -t <ip address of the client computer>

and see after how many packets a packet is dropped

what do u use a switch or a hub if u use a hub you migh exp. weak connectivity however it should be ok with a switch.

and IanTh is right u should use a repeater or a swicth in between afetr 75 metres or even 50 metres if possible.

or use stp a better option

yes and another though shielded stp may be a bit better

I use 75 meter when i ping sometime package lose 25 percent thiscame from cable or not

try a shorter cable or put a hub in between and then ping and c the packte loss